Low Mass Ratio Research Articles

Spectroscopic and photometric study for five late G- to K-type short-period contact binaries

We reported spectroscopy and photometry for five ultra-short period W UMa-type contact binaries. Eleven sets of low resolution spectra are obtained from the Large Sky Area Multiobject Fiber Spectroscopic Telescope (LAMOST) database and analyzed. It has been determined that these systems are late G- to K-type stars: G7V for J0658, G9V for LINEAR 3562115, K0V for PS Com, and K5V for LINEAR 11570575. Additionally, it was observed that the cooler the star, the stronger the sodium absorption line. Seven sets of complete light curves, including two sets of TESS data for PS Com and V0568 Peg, were analyzed using the W-D code. All the five targets are W-subtype systems with mass ratios (q) greater than 1. The light curves of the two medium contact systems (J0658 and LINEAR 11570575) are symmetric. In contrast, the light curves of the three shallow contact binaries (LINEAR 3562115, PS Com, and V0568 Peg) show differences between their two maxima, Max.I and Max.II, suggesting the presence of cool or hot spots on their surfaces. According to the H-R diagram, the more massive components of these targets are situated between the ZAMS and TAMS, while the less massive ones are overheated due to energy transfer within the common envelope. As these binaries lose angular momentum, they will evolve into deep, low mass ratio contact binaries.

Numerical Prediction of Two-Dimensional Coupled Galloping and Vortex-Induced Vibration of Square Cylinders Under Symmetric/Asymmetric Flow Orientations

This study presents an advanced numerical model for predicting a two-dimensional coupled galloping and vortex-induced vibration (VIV) in cross-flow and in-line directions of square cylinders under symmetric and asymmetric flow orientations. The present model combines the quasi-steady theory for the galloping with the nonlinear structure-wake oscillators simulating VIV, capturing the time-varying drag and lift hydrodynamic forces with the time-averaged and fluctuating components. By placing a flexibly mounted square cylinder in uniform flow at an initial angle of incidence, the cylinder is subject to instantaneous changes in the dynamic angle of attack accounting for relative flow-structure velocities. Modelling of such features in cross-flow and in-line directions for low and high mass ratio systems extends previous studies which have mostly focused on cross-flow responses of square cylinders with high mass ratios at a zero angle of incidence. New sets of empirical coefficients governing the drag and lift fluid forces for both the quasi-steady and wake oscillator approaches are introduced by calibrating with available experimental data in the literature, applicable to predict several flow-induced vibration phenomena under arbitrary flow-structure orientations. Mathematical criteria for the onset of two- and one-dimensional galloping instability are presented, verifying the likelihood of galloping occurrence. Parametric investigations are carried out to highlight the important effects of flow incidence angle, mass-damping ratio (Scruton number) and in-line response on the prediction of galloping and VIV in comparison with experimental results. By varying the reduced velocity parameter, the present model captures key qualitative features of the dominant galloping, interfering galloping-VIV and dominant VIV through the response amplitudes, mean drift displacements, oscillation frequencies, fluid force components and motion trajectories. Contributions from in-line responses are found to be meaningful for the interfering galloping-VIV system with a low mass-damping ratio and for an asymmetric flow orientation. The present model could be further calibrated and applied to other fluid-structure interaction applications with non-circular cross-sectional geometries under omnidirectional flow directions.

Three-Dimensional Flutter Numerical Simulation of Wings in Heavy Gas and Transonic Flutter Similarity Law Correction Method

Wind tunnel testing is a crucial method for studying aircraft flutter. Using heavy gas as the wind tunnel medium can mitigate the escalating issue of test models being overweight as advanced aircraft develop. This paper employs an analytical method for numerical calculations of three-dimensional (3D) wing flutter based on fluid–structure interaction (FSI). Flutter calculations for the Goland wing are conducted, and the results in the air medium are consistent with the literature. In contrast, significant differences in flutter behavior are observed in the heavy gas R134a medium. Compared to air, when the model reaches a critical state in R134a, the incoming flow velocity is lower, the incoming flow density is approximately 3 to 5 times air, and the incoming flow dynamic pressure is about 1.1 to 1.2 times that of air. The correction of heavy gas flutter data is crucial for wind tunnel testing. This paper proposes a correction method based on the unsteady transonic flow similarity law proposed by Bendiksen under quasi-steady conditions. Attempts are made to revise relevant published wind tunnel tests and heavy gas flutter calculation results. The transonic flutter similarity law effectively explains the flutter similarity of rigid models in both heavy gas and air media. Still, it fails in cases with highly reduced frequencies and low mass ratios, such as those encountered with flexible wings.

Population of giant planets around B stars from the first part of the BEAST survey

Exoplanets form from circumstellar protoplanetary disks whose fundamental properties (notably their extent, composition, mass, temperature, and lifetime) depend on the host star properties, such as their mass and luminosity. B stars are among the most massive stars and their protoplanetary disks test extreme conditions for exoplanet formation. This paper investigates the frequency of giant planet companions around young B stars (median age of 16 Myr) in the Scorpius-Centaurus (Sco-Cen) association, the closest association containing a large population of B stars. We systematically searched for massive exoplanets with the high-contrast direct imaging instrument SPHERE using the data from the BEAST survey, which targets a homogeneous sample of young B stars from the wide Sco-Cen association. We derived accurate detection limits in the case of non-detections. We found evidence in previous papers for two substellar companions around 42 stars. The masses of these companions are straddling the sim 13 Jupiter mass deuterium burning limit, but their mass ratio with respect to their host star is close to that of Jupiter. We derived a frequency of such massive planetary-mass companions around B stars of $11_ $<!PCT!>, accounting for the survey sensitivity. The discoveries of substellar companions b and B happened after only a few stars in the survey had been observed, raising the possibility that massive Jovian planets might be common around B stars. However, our statistical analysis shows that the occurrence rate of such planets is similar around B stars and around solar-type stars of a similar age, while B-star companions exhibit low mass ratios and a larger semi-major axis.

Experimental Study on the Effect of Ammonia on Combustion and Emission Characteristics of a Spark Ignition Engine Fueled with Hydrogen.

Using hydrogen and its compounds as fuel is one of the key ways to achieve zero carbon emissions in internal combustion engines. Due to the difference in fuel properties of hydrogen and ammonia, mixing the two can make up for each other's shortcomings in combustion performance. Therefore, this paper studies the effects of ammonia addition on the combustion and emission characteristics of a SI commercial hydrogen engine, and studies the differences in these effects under different excess air ratios. The results indicate that increasing the ammonia mass ratio lengthens both flame development and combustion duration, causing a delay in the combustion center. Under stoichiometric conditions, combustion instability increases, peak cylinder pressure and heat release rate decrease significantly, and exhaust temperatures rise. With a higher ammonia mass ratio, brake-specific fuel consumption first decreases and then increases, reaching a minimum at a 0.4-0.6 ammonia mass ratio. Unburned ammonia and N2O emissions gradually increase, while NO X emissions initially rise and then fall, peaking at a low ammonia mass ratio. From the perspective of balancing emissions and fuel economy, for hydrogen-ammonia mixed combustion engines, a 0.6 ammonia mass ratio is recommended under stoichiometric conditions, while lean burn conditions allow for a higher ammonia mass ratio.

Low-mass contact binaries: orbital stability at extreme low mass ratios

ABSTRACT With the ever-increasing number of light-curve solutions of contact binary systems, an increasing number of potential bright red nova progenitors are being reported. There remains, however, only one confirmed event. In this study, we undertake a comprehensive review of the orbital stability of contact binary systems, considering the effects of the stellar internal composition (metallicity) and age on the evolution of the gyration radius and its effect on the instability mass ratio of contact binaries. We find that both metallicity and age have an independent effect on orbital stability, with metal-poor and older systems being more stable. The combined effects of age and metallicity are quite profound, such that for most systems with primaries of solar mass or greater, which are halfway or more through the main-sequence lifespans have instability mass ratios at levels where the secondary component would be below the hydrogen fusion mass limit. We find that from the currently available solutions we cannot confidently assign any system as unstable. Although we identify eight potential red nova progenitors, all have methodological or astrophysical concerns, which lowers our confidence in designating any of them as potential merger candidates.

An Experimental Study on Vortex-Induced Vibration Suppression of a Long Flexible Catenary Cable by Using Vibration Dampers

In this paper, an experimental study is conducted to investigate the effectiveness of vibration dampers in suppressing vortex-induced vibration in a long, flexible catenary cable with a low mass ratio. The dampers, consisting of two small, symmetric, lightweight pipes clamped to the cable, are sparsely deployed along the cable to shape the vibration characteristics. The experimental results demonstrate that dampers significantly reduce the vibration amplitude by up to 60% and axial tension by up to 61% at high flow velocities, effectively suppressing the cable vibration in perpendicular flow. In addition, it is observed that the in-line and cross-flow vibration frequencies are approximately equal when the dampers are applied. This behavior contrasts with the conventional undamped catenary cable, where the in-line vibration frequencies are double those of the cross-flow frequencies.

Exact Solution of Nonlinear TVMD Based on Maximizing Damping Enhancement Effect for Acceleration Response Control

The tuned viscous mass damper (TVMD) exhibits superior control effect and energy dissipation capability compared to the viscous damper (VD). Existing literature has primarily focused on the control of absolute acceleration response in structures by linear TVMD, with limited research on nonlinear damper for controlling structural performance and efficiency. This study proposes a methodology for determining the optimal parameters of linear TVMD. The closed-form solution of the nonlinear TVMD is derived through stochastic linearization based on the linear TVMD, aiming to minimize the damping ratio of TVMD by utilizing the damping enhancement effect of the system. For lightly damped structures, the closed-form solution remains a reliable method for accurately approximating TVMD design parameters. Compared to traditional fixed-point theory, the TVMD developed through the proposed methodology demonstrates superior control performance, especially in terms of absolute acceleration, with a smaller damping ratio and minimal deviation from the optimal state of the TVMD. Additionally, the study investigated the influence of the damping index on the optimal parameters and the efficacy of the TVMD in vibration control. It is observed that reducing the damping index can improve structural performance, particularly in long-period structures with small mass ratios, although it may reduce the efficiency of the TVMD. Notably, TVMDs with lower mass ratios maintain higher efficiency regardless of the structural natural period. The damping index does not affect the optimal frequency ratio or the mean square response. Ultimately, the effectiveness of the optimization method is displayed through the examination of response spectra following 22 real seismic events, showcasing its adaptability across various structural types. The comparison of the root-mean-square response between the VD and TVMD across different periods shows consistent results, with a maximum deviation of only 10%. The TVMD is highly effective at regulating the structural response, excelling particularly in controlling the absolute acceleration of long-period structures, where it reduces maximum absolute acceleration by 20% more than displacement. The proposed exact solution rapidly and precisely identifies the design parameters of nonlinear TVMD by the desired specifications, thereby offering theoretical guidance for practical engineering applications.

Study on the Application of Determinant-MTMD(TMDD) Vibration Reduction in Cable-Supported Pedestrian Suspension Bridge

In this study, multiple tuned mass dampers (MTMDs) were studied to understand their impact on the human-induced vibration response and comfort level of a pedestrian cable-supported suspension bridge. A spatial finite element model based on a specific engineering case was established. The dynamic characteristics of the bridge under human-induced loads were investigated, and its comfort level under human-induced vibrations was analyzed using the time-history method. Then, this study adjusted the design parameters of the dampers based on various optimal damper parameter expressions. Furthermore, the damping effectiveness of MTMD under different mass ratios (μ) was evaluated, and it was found that increasing the mass ratio significantly impacts damping performance. Finally, determinant-TMD (TMDD) was introduced, and a comparison between the damping effect, robustness, and performance of TMDD and MTMD was conducted. The results indicate that while increasing the mass ratio does not linearly affect maximum vibration acceleration, the damping effect increases initially and then stabilizes, with a damping rate converging at approximately 55%. However, with the TMDD approach, the maximum damping rate can reach approximately 70%, enhancing comfort levels from the “minimum CL3” achieved with MTMD to the “medium CL2” level. Additionally, while TMDD’s robustness is slightly inferior to MTMD at lower mass ratios, it demonstrates superior robustness at higher mass ratios.

Investigative study of five low mass ratio semi-detached binaries

Investigative study of five low mass ratio semi-detached binaries

Net primary productivity of Betula platyphylla woodland and the response of current-year shoots to vegetation degradation in Mongolian forest steppe

At the southern margin of forest steppe vegetation in Mongolia, Betula platyphylla woodlands occur on north-facing slopes and hilltops, where congeneric shrub Betula fusca often coexists. Woodland degradation results in B. fusca scrub with scattered B. platyphylla trees. We studied the structure and productivity of a B. platyphylla woodland and the current-year shoot allometry in the Hustai National Park of central Mongolia. For the B. platyphylla woodland examined, stand basal area and estimated aboveground biomass for stems ≥ 3 cm diameter were 3.9 m2 ha–1 and 13.3 Mg dry mass ha–1, respectively. Leaf area index was 0.93 indicating open canopy layer. Aboveground net primary productivity as the sum of coarse wood production and current-year shoot mass amounted to 1.89 Mg dry mass ha–1 year–1, which was high relative to small aboveground biomass. Analysis of current-year shoot allometry demonstrated that B. platyphylla had more xeromorphic shoots in the scrub, with smaller specific leaf area, lower mass ratio of leaf laminae to supporting parts, and smaller lamina mass against the sum of petiole basal area, than in the woodland. Our results indicate that current-year shoot allometry characterizes tree response to woodland degradation at the dry margin of forest steppe.

Signatures of low-mass black hole–neutron star mergers

ABSTRACT The recent observation of the GW230529 event indicates that black hole–neutron star binaries can contain low-mass black holes. Since lower mass systems are more favourable for tidal disruption, such events are promising candidates for multimessenger observations. In this study, we employ five finite-temperature, composition-dependent matter equations of state and present results from ten 3D general relativistic hydrodynamic simulations for the mass ratios $q = 2.6$ and 5. Two of these simulations target the chirp mass and effective spin parameter of the GW230529 event, while the remaining eight contain slightly higher mass black holes, including both spinning ($a_{\mathrm{ BH}} = 0.7$) and non-spinning ($a_{\mathrm{ BH}} = 0$) models. We discuss the impact of the equation of state, spin, and mass ratio on black hole–neutron star mergers by examining both gravitational-wave and ejected matter properties. For the low-mass ratio model we do not see fast-moving ejecta for the softest equation of state model, but the stiffer model produces on the order of $10^{-6}\,\mathrm{ M}_\odot$ of fast-moving ejecta, expected to contribute to an electromagnetic counterpart. Notably, the high-mass ratio model produces nearly the same amount of total dynamical ejecta, but yields 52 times more fast-moving ejecta than the low-mass ratio system. In addition, we observe that the black-hole spin tends to decrease the amount of fast-moving ejecta while increasing significantly the total ejected mass. Finally, we note that the disc mass tends to increase as the neutron star compactness decreases.

Precipitation and diameter affect wood decomposition both directly and indirectly via deadwood traits and position

Woody plants are important components of dryland ecosystems. Our understanding of wood decomposition in drylands, an important component of biogeochemical cycling, is poor compared to that in mesic ecosystems. To uncovering the complex interactive effects of the different key drivers, we studied the effects of precipitation, position (aboveground and belowground), wood size and litter quality of plant species (four to five local species and one widespread woody species) on woody litter decomposition rates along a precipitation gradient from 37 to 369 mm, spanning five dryland sites. Wood dry matter content (DMC) was a critical negative predictor of wood decomposition in water-limited ecosystems. Thicker woody litter had lower decomposition rates (k values) directly because of smaller relative surface exposure, and indirectly through higher wood DMC or lower bark mass ratio (bark mass divided by wood mass for a given branch length). Mean annual precipitation (MAP) increased the k values both directly, and indirectly by decreasing the wood DMC and increasing the bark mass ratio due to species turnover. The k values of buried woody litter were mostly two to three times higher than litter on the soil surface, but not different at the extremely arid site. A steeper slope of the relationship between overall woody litter quality (particularly wood DMC) or annual precipitation and k values was observed belowground than aboveground, as related to the higher moisture belowground than aboveground. These findings highlight the complex interactions among climate (precipitation), litter position, size and quality on wood decomposition in drylands, thereby helping to improve our mechanistic understanding of dryland woody litter decomposition. We conclude that wood decomposition at the regional and local scales will influence biogeochemical cycling in drylands under future climate change through both direct effects of moisture and indirect effects of litter quality characteristics.

BSN: The First Light Curve Analysis of the Total Eclipse Binary System EL Tuc

We conducted the first light curve study of the binary star EL Tuc within the Binary Systems of South and North project’s framework. The photometric observations were made using standard multiband BVR c I c filters at an observatory in Argentina. We presented a new ephemeris for EL Tuc and a linear fit to the O – C diagram, utilizing our extracted times of minima and additional literature. We employed the PHysics Of Eclipsing BinariEs Python code and the Markov chain Monte Carlo approach for the system’s light curve analysis. The target system’s light curve solution required a cold starspot on the hotter component. We conclude that EL Tuc is a total contact binary system with a low mass ratio of q = 0.172 ± 0.002, an orbital inclination of i = 83.°74 ± 0.°40, and a fillout factor of f = 53.7% ± 1.6%. We used the P–a relationship and the Gaia Data Release 3 parallax method to determine the absolute parameters of EL Tuc to compare the precision of our results. This system was classified as W-type based on the mass and effective temperature of the companion stars. The positions of the systems were depicted on the M–L, M–R, T–M, and q–L ratio diagrams. The relationship between the spectroscopic and photometric mass ratios of binaries was discussed.

Analysis of KIC 7023917: Spotted Low-mass Ratio Eclipsing Binary with δ Scuti Pulsations

Times of minima of eclipsing binary KIC 7023917 show quasiperiodic anti-symmetric deviations from the calculated one with an amplitude of up to 10 minutes and a period of 200–300 days. These changes correlate with the observed variations of the light-curve maxima (amplitude and phase separation). We used photometric data obtained by Kepler and TESS missions to analyze the times of minima and determine system parameters. The phases and amplitudes of the maxima were measured to study the O’Connell effect. As an additional source of information, we performed ground-based multicolor photometric observation and determined the radial velocities of the system from our spectroscopic measurements. We could explain long-term variations of the light-curve shape and times of the eclipses using the cold star spot located on the secondary component and the modification of its size. Based on our modelling, the system consists of a primary main-sequence star of spectral type A7 and an evolved, oversized secondary component with a mass ratio of only 0.1 due to past mass transfer. Calculation of absolute parameters gives us the mass of the primary component about 1.8 M ☉ and 0.2 M ☉ for the secondary one, and radii of 2.2 R ☉ of the primary star and 0.9 R ☉ of secondary one, respectively. The studied low-mass ratio eclipsing binary is probably a progenitor of the variable star of EL CVn type. A multiple-period photometric variability was disclosed in the TESS data ranging from half to two hours due to δ Scuti-type pulsations of the primary component.

Detection of Two Totally Eclipsing B-type Binaries with Extremely Low Mass Ratios

The detection of O- and B-type stars with extremely low-mass companions is very important for understanding the formation and evolution of binary stars. However, finding them remains a challenge because the low-mass components in such systems contribute such small flux to the total. During our search for pulsations among O- and B-type stars using the Transiting Exoplanet Survey Satellite (TESS) data, we found two short-period and B-type (B9) eclipsing binaries with orbital periods of 1.61613 and 2.37857 days. Photometric solutions of the two close binaries were derived by analyzing the TESS light curves with the Wilson–Devinney method. It is discovered that both of them are detached binaries with extremely low mass ratios of 0.067(2) for TIC 260342097 and 0.140(3) for TIC 209148631. The determined mass ratio indicates that TIC 260342097 is one of the lowest mass ratios among known B-type binary systems. We showed that the two systems have total eclipses with a broad and flat secondary minimum, suggesting that the photometric parameters could be derived reliably. The absolute parameters of the two binaries are estimated and it is found that the secondary components in the two systems are overluminous and oversize when compared with the normal low-mass and cool main-sequence (MS) stars. These findings may imply that the two systems are composed of a B-type MS primary and a cool pre-MS secondary with orbital periods shorter than 2.5 days. They are valuable targets to test theories of binary star formation and evolution.

Design and optimization of inertial amplifier for enhanced vibration control of bridges under moving loads

This paper introduces a vibration control strategy for bridges that involves the implementation of an inertial amplifier to mitigate train-induced vibrations. The study comprehensively evaluates the effectiveness of two types of vibration absorbers, namely spring-mass resonator (SMR) and inertial amplifiers (IA), using a non-dimensional mechanics-based framework. The study further employs a heuristic search adaptive genetic algorithm (GA) to determine the optimal design parameters for the proposed vibration absorbers. The aim is to minimize the mid-span displacement of the bridge through the optimization process. The theoretical non-dimensional framework and the optimization technique are first validated with existing literature, and further, the efficiency of the optimized IA over the SMR of the same static mass is estimated. The comparative studies elucidate that with a lower mass ratio and higher values of the speed parameter (η) and inter-spatial distance between loads (ϵ), the IA system is more effective in reducing the vibration amplitude due to significant mass amplification. However, for lower values of η and ϵ, the SMR seems effective with a higher mass ratio, consequently resulting in amplified static deflection.

Enhancement of TiO2‐Based Composite With Low Carbon‐Based Component Ratio for Improved Hydrogen Generation via Photocatalytic Water Splitting

AbstractComposite between titanium dioxide (TiO2) and (reduced) graphene oxide (R(GO)) was prepared using a two‐stage solvothermal synthesis with variable R(GO) mass ratios (0.01–5 wt.%). Partial reduction of the precursor solution of GO to RGO took place during the solvothermal synthesis at the elevated pressure and temperature conditions. The structural, morphological, and semiconducting characteristics of the obtained binary composites were determined and their capacity of hydrogen production via photocatalytic water splitting in the presence of triethanolamine (TEOA) as sacrificial agent under the simulated solar light irradiation was tested. Photocatalytic experiments have showed that even low mass ratios of R(GO) component (below 1 wt.%) can have a great influence on the photocatalytic activity and properties of the obtained material. The results showed that even a partial reduction of GO to RGO had a positive impact on the photocatalytic properties of the as‐prepared materials. The composite with 0.05 R(GO) wt.% achieved the highest H2 generation rate of 139 μmol/h/g and maintained high photostability. The incorporation of R(GO) into the TiO2 matrix enhanced efficient charge separation, reduced the energy bandgap (Eg), and thus increased the visible light response (ΔE), leading to more effective hydrogen production.

Photometric and Spectroscopic Study of Ten Low Mass Ratio Contact Binary Systems: Orbital Stability, O’Connell Effect and Infrared Calcium Line Filling

Low mass ratio contact binary systems are more likely to have unstable orbits and potentially merge. In addition, such systems exhibit characteristics such as starspots and high energy emissions (UV) suggestive of chromospheric and magnetic activity. Light curve modeling of ten contact binary systems is reported. All were found to be of extreme low mass ratio ranging from 0.122 to 0.24 and three were found to be potentially unstable and possible merger candidates. Filling of the infrared calcium absorption lines is a marker of increased chromospheric activity. We use the available Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectra along with matched standard spectra (broadened for rotation) to measure the excess filling of the central core depression flux of the two main infrared calcium absorption lines λ8542 and λ8662. We find that all reported contact binaries have excess filling of the core flux in the infrared calcium lines. Three of the systems reported were also observed by the Galaxy Evolution Explorer mission and we find that all three have features of excess ultraviolet emissions further adding evidence for increased chromospheric activity in low mass ratio contact binaries. Analysis of both orbital stability and absorption line filling is dependent on the determination of geometric and absolute parameters from light curve modeling. Not an insignificant number of contact binary light curves exhibit the O’Connell effect, usually attributed to starspots. We discuss the inclusion of starspots in light curve solutions and how they influence the geometric and absolute parameters.

OO Leo: An Active Contact Binary with Possible Solar-like Differential Rotation

With Transiting Exoplanet Survey Satellite (TESS) high-precision photometry and Large Sky Area Multi-object Fiber Spectroscopic Telescope medium-resolution spectra, we present the first light and radial velocity curve analyses for the eclipsing binary OO Leo. The simultaneous solution suggests that OO Leo is a W-subtype contact binary with a relatively low mass ratio (1/q = 0.173) and a moderate degree of contact (f = 28.1%). The asymmetry and continuous changes observed in the TESS light curve were properly modeled by one retrograde cool spot on its secondary surface. A detailed investigation of the Hα line also confirmed that the secondary star had a high level of magnetic activity. The retrograde longitudinal motion of the spot can be explained by a solar-like differential rotation in the secondary component. The orbital period investigation revealed that OO Leo is undergoing a secular decrease and a cyclic variation in its orbital period. The secular decrease may be mainly caused by mass transfer from the more massive secondary star to the less massive primary star. The cyclic period variation can be explained by the light–time effect of an invisible third body or the cyclic magnetic activity of the secondary star. The long-lived spot migration in the longitudinal direction makes OO Leo an excellent target for investigating the differential rotations of contact binaries.