Insights into the Evolution of Low Mass-Ratio Contact Binaries: Orbital Period Analysis of 37 Systems

This study broadens our comprehensive investigation of total-eclipse W Ursae Majoris-type contact binaries by analyzing eight additional systems, continuing our previous research. Multiband BV R c I c photometric data were obtained at an observatory in Mexico, from which new times of minima were determined. All target systems also had available space-based Transiting Exoplanet Survey Satellite time-series data. Orbital period variations were studied for eight target systems, showing either linear or parabolic trends. The target systems exhibiting parabolic trends demonstrated a sustained decrease in their orbital periods over time. We modeled the light curves utilizing the PHOEBE Python code in combination with the BSN application. We revisited the relationship between orbital period and the temperature of the hotter component in contact binary systems using an empirical approach. Our analysis identified a clear break at P = 0.27 day, separating the systems into two distinct groups for orbital periods shorter than 0.6 day. Following the determination of stellar extinction, absolute parameters for seven systems were estimated employing parallax measurements from Gaia DR3. Based on the components’ effective temperatures and masses, the systems were classified into A- and W-subtypes. Their evolutionary states were illustrated using mass–radius and mass–luminosity diagrams.

The photometric and spectroscopic studies of six contact binaries were performed for the first time. The orbital periods of all the six targets are longer than 0.5 d, and we discovered that their mass ratios are smaller than 0.15. So, they are extremely low mass-ratio contact binaries. Only one target is a W-subtype contact binary (ASASSN-V J105032.88+420829.0), while the others are A-subtype contact binaries. From orbital period analysis, ASASSN-V J075442.44+555623.2 shows no orbital period change. Three of the six targets demonstrate a secular period increase and two targets for a secular period decrease. We investigated the LAMOST spectra employing the spectral subtraction method. All six contact binaries show no chromospheric emission line, implying no chromospheric activity. Their absolute parameters, initial masses, ages, energy transfer parameters, and instability parameters were calculated. The bolometric luminosity ratios ($(L_2/L_1)_{\mathrm{ bol}}$), the energy transfer parameters ($\beta$), the contact degrees (f), and the mass ratios (q) were collected for a sample of 218 contact binaries and we analysed and discussed some correlations. The results by analysing the relation between $\beta$, f, and q indicate that the energy transfer parameter between the two components of extremely low mass-ratio contact binaries is independent of the contact degree. And the predicted cut-off mass ratio was estimated as 0.021 by analysing the relation between f and q.

This study presents a detailed photometric and spectroscopic analysis of the W UMa-type binary NR Cam, using data from the Transiting Exoplanet Survey Satellite (TESS) and ground-based observations. The light curves exhibit significant variable, with a negative correlation between the brightness of the two maxima—a characteristic of W UMa-type binaries typically attributed to magnetic activity. To explain this behavior, we incorporated a starspot model into our Wilson–Devinney analysis. Our results confirm that NR Cam is a W-subtype, moderately contact binary with a low mass ratio of q = 5.75(±0.03) and a fill-out factor of f = 33.4(±3.1)%. We also analyzed the orbital period variation using all available times of minima. The resulting O − C diagram reveals a long-term decreasing trend in the orbital period at a rate of dP/dt = −5.18(±0.02) × 10−8 day yr−1, superimposed with a periodic oscillation characterized by an amplitude of A 3 = 0.0019(±0.0001) day and an oscillation period of P 3 = 7.776(±0.003) yr. The long-term decrease is likely due to mass transfer between the binary components, with an estimated mass transfer rate of dM 2/dt = 1.33(±0.01) × 10−8 M ⊙ yr−1. The periodic oscillations are likely driven by the light-travel time effect caused by a tertiary companion, with a minimum mass of M 3 = 0.0956(1)M ⊙ and a maximum separation of 3.841(6) au. Additionally, we considered the possibility that the periodic variation could result from changes in the gravitational quadrupole moment due to magnetic activity cycles, as described by the Applegate mechanism. Our findings confirm that NR Cam is an active binary system, where magnetic activity plays a significant role in its orbital evolution. These results contribute to our understanding of the magnetic dynamics and evolutionary processes in contact binary systems.

W UMa-type contact binaries belong to close binary systems whose components exactly overflow their Roche lobes and share a common convective envelope (CCE). In the last twenty years, the long-term variations of their orbital periods have been thought to depend on several mechanisms. Now, we suggest a new mechanism: CCE-dominated mechanism. The CCE-dominated mechanism is found based on our numerical result, especially at high mass ratios, that the orbital periods (P) of contact binaries change very much with their fill-out factors (f). Because f is taken as a measurement of the thickness of CCE, the physical cause for the variation of P is a mass transfer between CCE and components. Further, an f-dominated simplification model for this mechanism is introduced. According to it, P may change in a long-term oscillation way with a similar time scale of the thermal modulation, meanwhile q is decreasing slowly till the two components merge. It could be also applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. Moreover, the CCE-dominated mechanism should always work due to mass transfer and mass loss both occurring via CCE. Therefor, the effect of CCE on the variations of orbital periods may have been underestimated before.

A comprehensive photometric study and an investigation of the orbital period variation of V53 in the globular cluster M 4 are presented. The photometric study reveals that the mass ratio and the contact degree of V53 are q ∼ 0.078 and f ∼ 69%, respectively. The observed variation in the light curve can be explained by adjusting the spot parameters. V53 belongs to extreme mass ratio (q ≤ 0.25), deep contact (f ≥ 50%) binaries, and its mass ratio is close to the minimum mass ratio predicted by theoretical studies, making it a potential object for studying the evolution of binaries and the formation of blue stragglers and FK Com-type stars. The orbital period of V53 shows a long-term decrease at a rate of dp/dt = 5.89(±0.02) × 10−8 d yr−1. This secular period decrease may be caused by the combination of mass transfer from the more massive component to the less massive component and an angular momentum loss via magnetic braking. As this mass transfer and angular momentum loss continues, V53 will ultimately evolve into a single fast-rotation star. By studying the statistics of all the contact binaries in globular clusters that have been analyzed, we found a possible correlation between the contact degree and whether or not a contact binary is a blue straggler. A contact binary is likely to become a blue straggler when its fill-out factor is more than 46.25(±2.05)%. More samples should be introduced to confirm this preliminary result in the future.

This study continues our in-depth investigation of total-eclipse W Ursae Majoris-type contact binaries by analyzing eight new systems, complementing our previous work. Multiband BVR c I c photometric data were acquired through ground-based observations at an observatory in Mexico, from which new times of minima were determined. Our analysis of orbital period variations using the O − C method revealed that one system shows no long-term variation, four systems exhibit a secular decrease in their orbital periods, and two systems exhibit a secular increase, suggesting mass transfer between the components. Notably, one system displays a cyclic variation with an amplitude of 0.00865 days and a period of 10.49 yr, which we attribute to the light travel time effect induced by a tertiary companion, possibly a brown dwarf. We modeled the light curves using the PHysics Of Eclipsing BinariEs Python code. Six of the target systems required the inclusion of a cold starspot on one of the system’s stars due to the asymmetry observed in the maxima of their light curves. Absolute parameters were estimated using the Gaia DR3 parallax method. Using the components’ effective temperatures and masses, we classified five of the systems as W-subtype and three as A-subtype. The stellar evolution was illustrated through the mass–radius and mass–luminosity diagrams. Furthermore, we investigated the dynamical stability of two systems with extremely low mass ratios.

We collected photometric data from the Transiting Exoplanet Survey Satellite and spectroscopic observations from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. Using this data, we simultaneously analyzed the radial velocity and light curves of four eclipsing contact binaries, i.e. ASAS J061658+1901.2, NSVS 4919378, V589 Lyr, and V1188 Tau, to accurately determine their absolute physical parameters. Based on the eclipse times obtained from the observations of various surveys, we investigated their orbital period variations, finding that the orbital periods of ASAS J061658+1901.2 and NSVS 4919378 decrease and increase continuously, respectively. Additionally, changes in secular period are likely to occur because of mass transfer between the two components; however, the possibility of long-term period changes caused by a third body cannot be excluded. The orbital period of V589 Lyr has remained stable over the past two decades, whereas V1188 Tau has exhibited a cyclic change in its orbital period. This may be attributed to either the light–time effect of a third body or the cyclic magnetic activity of its components. A study of the distribution of total mass, temperature difference, orbital angular momentum, and orbital period suggests that W-subtype contact binaries are more evolved than A-subtype ones.

We presented the first photometric and orbital period investigations for four W Ursae Majoris-type binaries: V473 And, V805 And, LQ Com, and EG CVn. The photometric solutions suggested that V805 And and LQ Com are two total-eclipse contact binaries, while V473 And and EG CVn are partial-eclipse ones. V473 And and LQ Com belong to the A-subtype contact binaries, while V805 And and EG CVn belong to the W subtype. The O’Connell effects found in the light curves of V805 And, LQ Com, and EG CVn can be interpreted as a result of a cool spot on the surface of their less massive and hotter primary components. Based on two different methods, the absolute physical parameters were properly determined. Combining the eclipse timings derived from our observations and survey’s data with those collected from literature, we investigated their orbital period variations. The results show that the orbital periods of V473 And, V805 And, and EG CVn are undergoing a secular decrease/increase superposed a periodic variation, while LQ Com exhibits a possible cyclic period variation with a small amplitude. The secular period changes are caused mainly by the mass transfer between two components, while the cyclic period oscillations may be interpreted as the results of either the light-time effect due to the third body or the cyclic magnetic activity. Finally, we made a statistical investigation for nearly 200 contact binaries with reliable physical parameters. The statistical results suggested that the W-subtype systems are more evolved than the A-subtype ones. Furthermore, the evolutionary direction of A-subtype into W-subtype systems is also discussed. The opposite evolutionary direction seems to be unlikely because it requires an increase of the total mass, the orbital angular momentum, and the temperature differences between two components of a binary system.

The period distribution of contact binaries exhibits a very sharp short period cut-off at 0.22 days. In order to provide valuable information on this short period limit, we observed ten totally eclipsing contact binaries with orbital periods near this cut-off. By detailed analysis using W-D code, we determined that two of these systems are A-subtype contact binaries while the others are W-subtype contact binaries and all the targets show shallow contact configurations. Half of the targets exhibit stellar spot activity and four of them have third light. A statistical work on well studied USPCBs was carried out, and physical parameters of fifty-five USPCBs were obtained. Some common properties for these systems were derived. Due to the study of the period-color diagram of USPCBs, we found that the period-color relation of USPCBs is different from other W UMa type contact binaries and USPCBs are metal poor old stars. In addition, the evolutionary states of these systems were discussed by constructing the color-density diagram. We derived that the evolutionary states of the two components of USPCBs show identical characteristics of other contact binaries despite slower evolutionary status caused by smaller mass. We suggested that both the fully convective limit claimed by Rucinski (1992) and the AML theory recommended by Stepien (2006) can produce the short period cut-off and a tertiary companion is very important during the formation of the short period contact binaries by removing angular momentum from the host eclipsing pairs under certain circumstances.

We present a homogeneously selected sample of 15 779 candidate binary systems with main sequence primary stars and orbital periods shorter than 5 d. The targets were selected from TESS full-frame image light curves on the basis of their tidally induced ellipsoidal modulation. Spectroscopic follow-up suggests a sample purity of 83 ± 13 per cent. Injection-recovery tests allow us to estimate our overall completeness as 28 ± 3 per cent with Porb < 3 d and to quantify our selection effects. 39 ± 4 per cent of our sample are contact binary systems, and we disentangle the period distributions of the contact and detached binaries. We derive the orbital period distribution of the main-sequence binary population at short orbital periods, finding a distribution continuous with the lognormal distribution previously found for solar-type stars at longer periods, but with a significant steepening at Porb ≲ 3 d, and a pile-up of contact binaries at Porb ≈ 0.4 d. Companions in the period range of 1–5 d are an order of magnitude more frequent around stars hotter than $\approx 6250\, \rm K$ (the Kraft break) when compared to cooler stars, suggesting that magnetic braking shortens the lifetime of cooler binary systems. However, the period distribution in the range 1–10 d is independent of temperature. We detect resolved tertiary companions to 9.0 ± 0.2 per cent of our binaries with a median separation of 3200 au. The frequency of tertiary companions rises to 29 ± 5 per cent among the systems with the shortest ellipsoidal periods. This large binary sample with quantified selection effects will be a powerful resource for future studies of detached and contact binary systems with Porb<5 d.

Orbital period variations of two contact binaries DF Hya and WZ And are analyzed with the least-squares method by using all available minima times. It is shown that the period variations of these systems are due mainly to the Light-Time Effect (LITE) due originates from gravitational influence of a third body. New LITE elements such as, orbital periods and minimum masses of possibility third bodies are given.

Period changes in six contact binaries: WZ And, V803 Aql, DF Hya, PY Lyr, FZ Ori, and AH Tau

The study of a possible connection between magnetic activity and orbital period variation in close binaries is a very interesting work. Recently, the orbital periods of four chromospherically active binaries, ER Vul, UV Psc, AR Lac and BH Vir, are analyzed. It is discovered that the orbital periods of UV Psc and BH Vir oscillate with periods of 61 and 9.12 years, and the orbital periods of ER Vul and AR Lac show periodic variations with periods of 31 and 47 years respectively while they undergo secular decrease. The mechanisms that could explain the changes in the orbital periods of the four systems have been studied. The period variation of UV Psc may be caused by the cyclical magnetic activity in the primary component, and the magnetic activity in secondary component of AR Lac can explain its periodic component in the orbital period changes. For the other two systems, BH Vir and ER Vul, the cyclical magnetic activity in one or both of the components can explain the cyclical orbital period changes of BH Vir and the periodic component in the changes of the orbital period of ER Vul. These results suggest that the periods of the orbital period oscillations in the four systems may be the magnetic activity cycles.

Orbital period changes of eight W-type contact binaries (TY Boo, BH Cas, AD Cnc, TX Cnc, RZ Com, LS Del, BB Peg and AA UMa) are presented based on the analysis of their O–C curves. It is found that the periods of the five systems TY Boo, TX Cnc, RZ Com, LS Del and AA UMa show secular increase. For BB Peg, its period increase rate has been revised. For AD Cnc, weak evidence also shows that its orbital period is increasing. For the remaining BH Cas, the three times of light minimum given by Agerer & Hubscher indicate that, recently, its period has been increasing. However, the properties of the period need further study. The mass ratios of all the systems are larger than 0.4. The period increases of the systems may suggest that the W-type W UMa stars with high mass ratio usually show their period increase. In order to check this conclusion, secular period changes of 30 W-type contact binaries have been collected from the literature. It is found that systems showing period increase usually have a higher mass ratio , and the periods of low-mass ratio systems are varying in a secular decrease. This strongly suggests that a relation between the orbital period variation and the mass ratio for W-type contact binaries may exist. If the secular period change is caused by conservative mass transfer between the components, this relation may suggest that the evolution of the W-type systems is oscillation around a critical mass ratio . However this is highly speculative. On the other hand, the relation may be potentially strong observational evidence for Rahunen's conclusion that angular momentum loss (AML) may enable the components of a contact binary to remain in good contact throughout the thermal relaxation oscillation (TRO) cycle. This connection could be explained by the combination of the TRO and the variable AML via the change of depth of contact, which needs further studies observationally and theoretically.

Since the data obtained from satellite data are very sensitive, the minima times of the target systems have obtained from Kepler (K2) and TESS observations. A total of 14 minima times were obtained from Kepler (K2) data for the AF Gem system and 11 minima times in two different sectors from the TESS data for the RY Gem system. In this study, orbital period behaviors of semi-detached binaries (SDBs) AF Gem and RY Gem systems have investigated, together with minima times obtained from literature and satellite data. It has been observed that the orbital periods of AF Gem and RY Gem decrease. The period change rates of AF Gem and RY Gem systems are determined as -4.1×10-8 and -6.5×10-6 day/ year. The magnetic activity of the cooler components of the systems may be the reason for the periods decrease, and therefore the mass loss rates of the systems have calculated. For AF Gem and RY Gem, mass loss rate was found to be -4.4×10-8 M⊙/year and -8.2×10-7 M⊙/year, respectively. In the O-C graph of AF Gem, sinüsoidal variations also exist. Sinusoidal change may be demonstrated as being the result of a light time effect (LITE) via a tertiary body around the eclipsing pair. The minimum mass of probable third star around AF Gem has found to be 0.24 M⊙.