Ks-band Photometry Research Articles

Projection factor and radii of Type II Cepheids

Context. Type II Cepheids are old pulsating stars that can be used to trace the distribution of an old stellar population and to measure distances to globular clusters and galaxies within several megaparsecs, and by extension, they can improve our understanding of the cosmic distance scale. One method that can be used to measure the distances of Type II Cepheids relies on period-luminosity relations, which are quite widely explored in the literature. The semi-geometrical Baade-Wesselink technique is another method that allows distances of radially pulsating stars, such as Type II Cepheids, to be measured if the so-called projection factor is known. However, the literature concerning this parameter for Type II Cepheids is limited to just a few pioneering works. Aims. In determining projection factors for eight nearby short-period Type II Cepheids, also known as BL Her type stars, we aim to calibrate the Baade-Wesselink method for measuring distances for this class of stars. Methods. Using the surface brightness-colour relation version of the Baade-Wesselink technique, we determined the projection factors and radii of eight nearby BL Her type stars. We adopted accurate distances of target stars from Gaia Data Release 3. Time series photometry in the V and KS bands have been collected with two telescopes located at the RolfChini Cerro Murphy Observatory (former Cerro Armazones Observatory), while spectroscopic data have been obtained within dedicated programmes with instruments hosted by the European Southern Observatory. Results. The measured projection factors for the stars with good quality data are in the range between 1.21 and 1.36. The typical uncertainty of projection factors is 0.1. The mean value is 1.330 ± 0.058, which gives the uncertainty of ∼4%. The main sources of uncertainty on the p-factors are statistical errors of the Baade-Wesselink fit (related to the dispersion and coverage of light and radial velocity curves) and parallax. In the case of radii, the biggest contribution to the error budget comes from the KS band photometry’s systematic uncertainty and parallax. The determined radii allowed us to construct the period-radius relation for BL Her stars. Our period-radius relation is in good agreement with the previous empirical calibration, while two theoretical calibrations found in the literature agree with our relation within 2σ. We also confirm that BL Her and RR Lyr stars obey an apparent common period-radius relation.

The Distance and Peculiar Velocity of the Norma Cluster (ACO3627) Using the Near-infrared J- and K s -band Fundamental Plane Relations

Abstract We report distance measurements for the Norma cluster based on the near-infrared J- and K s -band fundamental plane (FP) relations. Our simultaneous J- and K s -band photometry analyses were performed using 31 early-type galaxies in the nearby Norma cluster obtained using the 1.4 m InfraRed Survey Facility (IRSF) at the South African Astronomical Observatory. Our final K s -band FP sample consists of 41 early-type galaxies from the Norma cluster observed using the IRSF and the New Technology Telescope (NTT) at the European Southern Observatory. This is the largest cluster sample used for peculiar velocity studies in the Great Attractor region to date. From the K s -band FP, we find a distance to the Norma cluster of 4915 ± 121 km s−1. The implied peculiar velocity for Norma is 44 ± 151 km s−1, which further supports a small peculiar velocity for the Norma cluster.

MAGELLAN AO SYSTEM z′, YS, AND L′ OBSERVATIONS OF THE VERY WIDE 650 AU HD 106906 PLANETARY SYSTEM*

ABSTRACT We analyze archival data from Bailey and co-workers from the Magellan adaptive optics system and present the first 0.9 μm detection (z′ = 20.3 ± 0.4 mag; Δz′ = 13.0 ± 0.4 mag) of the 11 M Jup circumbinary planet HD 106906AB b, as well as 1 and 3.8 μm detections of the debris disk around the binary. The disk has an east–west asymmetry in length and surface brightness, especially at 3.8 μm where the disk appears to be one-sided. The spectral energy distribution of b, when scaled to the K S -band photometry, is consistent with 1800 K atmospheric models without significant dust reddening, unlike some young, very red, low-mass companions such as CT Cha B and 1RXS 1609 B. Therefore, the suggested circumplanetary disk of Kalas and co-workers might not contain much material, or might be closer to face-on. Finally, we suggest that the widest (a ≳ 100 AU) low mass ratio (M p/M ⋆ ≡ q ≲ 0.01) companions may have formed inside protoplanetary disks but were later scattered by binary/planet interactions. Such a scattering event may have occurred for HD 106906AB b with its central binary star, but definitive proof at this time is elusive.

Optical-near-IR analysis of globular clusters in the IKN dwarf spheroidal: a complex star formation history

Age, metallicity and spatial distribution of globular clusters (GCs) provide a powerful tool to reconstruct major star-formation episodes in galaxies. IKN is a faint dwarf spheroidal (dSph) in the M81 group of galaxies. It contains five old GCs, which makes it the galaxy with the highest known specific frequency (SN=126). We estimate the photometric age, metallicity and spatial distribution of the poorly studied IKN GCs. We search SDSS for GC candidates beyond the HST field of view, which covers half of IKN. To break the age-metallicity degeneracy in the V-I colour we use WHT/LIRIS Ks-band photometry and derive photometric ages and metallicities by comparison with SSP models in the V,I,Ks colour space. IKN GCs' VIKs colours are consistent with old ages ($\geq\!8$ Gyr) and a metallicity distribution with a higher mean than typical for such a dSph ([Fe/H$]\!\simeq\!-1.4_{-0.2}^{+0.6}$ dex). Their photometric masses range ($0.5 <{\cal M_{\rm GC}}<4\times10^5M_\odot$) implies a high mass ratio between GCs and field stars, of $10.6\%$. Mixture model analysis of the RGB field stars' metallicity suggests that 72\% of the stars may have formed together with the GCs. Using the most massive GC-SFR relation we calculate a SFR of $\sim\!10M_\odot/$yr during its formation epoch. We note that the more massive GCs are closer to the galaxy photometric centre. IKN GCs also appear spatially aligned along a line close to the IKN major-axis and nearly orthogonal to the plane of spatial distribution of galaxies in the M81 group. We identify one new IKN GC candidate based on colour and PSF analysis of the SDSS data. The evidence towards i) broad and high metallicity distribution of the field IKN RGB stars and its GCs, ii) high fraction and iii), spatial alignment of IKN GCs, supports a scenario for tidally triggered complex IKN's SFH in the context of interactions with galaxies in the M81 group.

Long-termKs-band photometric monitoring of L dwarfs

(abridged) We perform photometric time-series analysis of a sample of ten early to mid-L dwarfs in the field over three years of $K_s$-band observations with the OMEGA 2000 infrared camera of the 3.5m telescope on Calar Alto Observatory between January 2010 and December 2012. We perform $K_s$-band differential photometry of our targets (with typical errors of $\pm$15-30~mmag at the 1$\sigma$ level) by subtracting a reference flux from each photometric measurement. This reference flux is computed using three nearby, probably constant stars in the target's field-of-view. We then construct and visually inspect the light curves to search for variability, and use four different periodogram algorithms to look for possible periods in our photometric data. Our targets do not display long-term variability over 1$\sigma$ compared to other nearby stars of similar brightness, nor do the periodograms unveil any possible periodicity for these objects, with two exceptions: 2MASS~J02411151-0326587 and G196-3B. In the case of 2MASS~J02411151-0326587 (L0), our data suggest a tentative period of 307$\pm$21~days, at 40% confidence level, which seems to be associated with peak-to-peak variability of 44$\pm$10~mmag. This object may also display variability in timescales of years, as suggested by the comparison of our Ks-band photometry with 2MASS. For G196-3B (L3), we find peak-to-peak variations of 42$\pm$10~mmag, with a possible photometric period of 442$\pm$7~days, at 95% confidence level. This is roughly the double of the astrometric period reported by Zapatero Osorio (2014). Given the significance of these results, further photometric data are required to confirm the long-term variability.These results suggest that early- to mid-L dwarfs are fairly stable in the $K_s$-band within $\pm$90 mmag at the 3 $\sigma$ level over months to years, which covers hundreds to tens of thousands of rotation cycles.

INTERMEDIATE-AGE GLOBULAR CLUSTERS IN FOUR GALAXY MERGER REMNANTS

We present the results of combining Hubble Space Telescope optical photometry with ground-based Ks-band photometry from the Gemini imagers NIRI and FLAMINGOS-I to study the globular-cluster populations in four early-type galaxies that are candidate remnants of recent mergers (NGC1700, NGC2865, NGC4382, and NGC7727). These galaxies were chosen based on their blue colors and fine structure, such as shells and ripples that are indicative of past interactions. We fit the combined VIKs globular-cluster data with simple toy models of mixed cluster populations that contain three subpopulations of different age and metallicity. The fits, done via Chi-square mapping of the parameter space, yield clear evidence for the presence of intermediate-age clusters in each galaxy. We find that the ages of 1-2 Gyr for these globular-cluster subpopulations are consistent with the previously estimated merger ages for the host galaxies.

A brown dwarf companion to the intermediate-mass star HR 6037

In the course of an imaging survey we have detected a visual companion to the intermediate-mass star HR 6037. In this letter, we present two epoch observations of the binary with NACO/VLT, and near-IR spectroscopy of the secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as a co-moving companion. Its J and H band ISAAC spectra suggest the object has an spectral type of M9+-1, with a surface gravity intermediate between that of 10 Myr dwarfs and field dwarfs with identical spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K, and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03, and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary system.

NEAR-INFRARED THERMAL EMISSION FROM TrES-3b: AKs-BAND DETECTION AND ANH-BAND UPPER LIMIT ON THE DEPTH OF THE SECONDARY ECLIPSE

We present H and Ks-band photometry bracketing the secondary eclipse of the hot Jupiter TrES-3b using the Wide-field Infrared Camera on the Canada-France-Hawaii Telescope. We detect the secondary eclipse of TrES-3b with a depth of 0.133+/-0.017% in Ks-band (8-sigma) - a result in sharp contrast to the eclipse depth reported by de Mooij & Snellen. We do not detect its thermal emission in H-band, but place a 3-sigma limit on the depth of the secondary eclipse in this band of 0.051%. A secondary eclipse of this depth in Ks requires very efficient day-to-nightside redistribution of heat and nearly isotropic reradiation, conclusion that is in agreement with longer wavelength, mid-infrared Spitzer observations. Our 3-sigma upper-limit on the depth of our H-band secondary eclipse also argues for very efficient redistribution of heat and suggests that the atmospheric layer probed by these observations may be well homogenized. However, our H-band upper limit is so constraining that it suggests the possibility of a temperature inversion at depth, or an absorbing molecule, such as methane, that further depresses the emitted flux at this wavelength. The combination of our near-infrared measurements and those obtained with Spitzer suggest that TrES-3b displays a near isothermal dayside atmospheric temperature structure, whose spectrum is well approximated by a blackbody. We emphasize that our strict H-band limit is in stark disagreement with the best-fit atmospheric model that results from longer wavelength observations only, thus highlighting the importance of near-infrared observations at multiple wavelengths in addition to those returned by Spitzer in the mid-infrared to facilitate a comprehensive understanding of the energy budgets of transiting exoplanets.

THEXMMCLUSTER SURVEY: THE BUILD-UP OF STELLAR MASS IN BRIGHTEST CLUSTER GALAXIES AT HIGH REDSHIFT

We present deep J and Ks band photometry of 20 high redshift galaxy clusters between z=0.8-1.5, 19 of which are observed with the MOIRCS instrument on the Subaru Telescope. By using near-infrared light as a proxy for stellar mass we find the surprising result that the average stellar mass of Brightest Cluster Galaxies (BCGs) has remained constant at ~9e11MSol since z~1.5. We investigate the effect on this result of differing star formation histories generated by three well known and independent stellar population codes and find it to be robust for reasonable, physically motivated choices of age and metallicity. By performing Monte Carlo simulations we find that the result is unaffected by any correlation between BCG mass and cluster mass in either the observed or model clusters. The large stellar masses imply that the assemblage of these galaxies took place at the same time as the initial burst of star formation. This result leads us to conclude that dry merging has had little effect on the average stellar mass of BCGs over the last 9-10 Gyr in stark contrast to the predictions of semi-analytic models, based on the hierarchical merging of dark matter haloes, which predict a more protracted mass build up over a Hubble time. We discuss however that there is potential for reconciliation between observation and theory if there is a significant growth of material in the intracluster light over the same period.

NEAR-INFRARED THERMAL EMISSION FROM THE HOT JUPITER TrES-2b: GROUND-BASED DETECTION OF THE SECONDARY ECLIPSE

We present near-infrared Ks-band photometry bracketing the secondary eclipse of the hot Jupiter TrES-2b using the Wide-field Infrared Camera on the Canada–France–Hawaii Telescope. We detect its thermal emission with an eclipse depth of 0.062+0.013−0.011% (5σ). Our best-fit secondary eclipse is consistent with a circular orbit (a 3σ upper limit on the eccentricity, e, and argument or periastron, ω, of |e cos ω| < 0.0090), in agreement with mid-infrared detections of the secondary eclipse of this planet. A secondary eclipse of this depth corresponds to a dayside Ks-band brightness temperature of TB = 1636+79−88 K. Our thermal emission measurement, when combined with the thermal emission measurements using Spitzer/IRAC from O'Donovan and collaborators, suggests that this planet exhibits relatively efficient dayside to nightside redistribution of heat and a near isothermal dayside atmospheric temperature structure, whose spectrum is well approximated by a blackbody. It is unclear if the atmosphere of TrES-2b requires a temperature inversion; if it does it is likely due to chemical species other than TiO/VO as the atmosphere of TrES-2b is too cool to allow TiO/VO to remain in gaseous form. Our secondary eclipse has the smallest depth of any detected from the ground, at around 2 μm, to date.

Metallicities and radial velocities of five open clusters including a new candidate member of the Monoceros stream

Near infrared spectra of 133 red giant stars from ten Galactic open clusters and two Galactic globular clusters spanning 2.2 dex in metallicity and 11 Gyr in age are presented. We combine this sample with ten clusters from Cole and collaborators to investigate the Ca II triplet line strengths and their relation to cluster metallicity and position along the red giant branch. We show that characterizing the stellar surface gravity using Ks band photometry (relative to the horizontal branch) taken from the Two Micron All-Sky Survey allows for metallicity measurements at least as precise as those derived using V or I band data. This has the great advantage that uniform photometry and reliable astrometry is available for a large number of clusters. Using Ks band photometry also reduces the effect of differential reddening within a given cluster. We find no significant evidence for age or metallicity effects to the linear Ca II triplet - metallicity relationship over the small range in magnitudes studied when homogeneous reference metallicities are used. We derive the first spectroscopic metallicity and new radial velocity estimates for five open clusters: Berkeley 81, Berkeley 99, IC 1311, King 2, and NGC 7044. King 2 has an anomalous radial velocity compared with the local disk population. We discuss the possibility that it is part of the Monoceros tidal stream.

On the accuracy of the high-redshift cluster luminosity function

We study the reliability of the statistical background subtraction method for computing the Ks-band luminosity function of cluster galaxies at z∼ 1 using mock Red-sequence Cluster Survey cluster catalogues constructed from galform semi-analytic galaxies. The underlying cluster luminosity function in the mocks is compatible with recent estimates at z∼ 1 by several authors. We simulate different samples where the number of clusters with Ks-band photometry goes from five to a maximum of 50, in order to find the most suitable observational sample to carry out this study; the current observational status in the near-infrared wavelength range has been reached using five real clusters at z∼ 1. We compute the composite luminosity function for several samples of galaxy clusters with masses ≃1.5 × 1014 M⊙ assuming a flux-limited, complete sample of galaxies down to Ks= 21.0 mag. We find that the Schechter fit parameters Ks* and α for a sample of galaxies with no redshift information are rather poorly constrained if both parameters are allowed to vary freely; if α is fixed at a fiducial value, then Ks* shows significantly improved stochastic uncertainties but can be influenced by systematic deviations. We find a significantly improved accuracy in the luminosity function parameters when adding photometric redshift information for bright cluster galaxies. The impact of a 10-fold increase in the number of clusters with available Ks-band photometry is that of decreasing stochastic errors in Ks* and α by factors of ≃2 and ≃4, respectively, for accuracies of up to (ΔKs*/Ks*)stochastic= 0.02 and (Δα/α)stochastic= 0.09. The dwarf-to-giant ratios inferred from the luminosity functions of red-sequence galaxies in the mock catalogue agree very well with the underlying values; however, there is an indication that the semi-analytic model overpredicts the abundance of dwarf galaxies by up to a factor of 3 with respect to recent measurements. Finally, we find that in order to use estimates of Ks* to study the formation redshift of cluster galaxies at z= 1, the sample would need to contain 520 z∼ 1 clusters, for an accuracy of ∼2 Gyr at the 68 per cent confidence level. However, combining this method with other estimates may significantly reduce the sample size, and allow important new constraints on galaxy formation models.

Young globular clusters in an old S0: clues to the formation history of NGC 4570

We here present our first attempt to use Globular Clusters as tracers of their parent galaxy’s formation history. Globular Cluster Systems of most early-type galaxies feature two peaks in their optical colour distributions. Blue-peak Globular Clusters are generally believed to be old and metal-poor. The ages, metallicities, and the origin of the red-peak Globular Clusters are being debated. We here present our analysis of the ages and metallicities of the red peak Globular Clusters in the Virgo S0 NGC 4570 using deep Ks-band photometry from NTT/SOFI (ESO program ID 079.B-0511) for the red-peak Globular Clusters in combination with HST-ACS archival data to break the age-metallicity degeneracy. We analyze the combined g, z, and Ks spectral energy distribution by comparison with a large grid of GALEV evolutionary synthesis models for star clusters with different ages and metallicities. This analysis reveals a substantial population of intermediate-age (1–3 Gyr) and metal-rich (� solar metallicity) Globular Clusters. We discuss their age and metallicity distributions together with information on the parent galaxy from the literature to gain insight into the formation history of this galaxy.

Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates. IV. Solution to the Puzzle of the Extremely Red OGLE‐TR‐82 Primary

We present precise new V-, I-, and Ks-band photometry for the planetary-transit candidate star OGLE-TR-82. V-band images acquired in good seeing with the VIMOS instrument at the Very Large Telescope allowed us to measure V = 20.61 ± 0.03 mag for this star despite the presence of a brighter neighbor about 1'' away. This faint magnitude answers the question why it has not been possible to measure radial velocities for this object. One transit of this star has been observed with the GMOS-S instrument on Gemini South in the i and g bands, which allowed us to verify that this is not a false positive, to confirm the transit amplitude measured by OGLE, and to improve the ephemeris. The transit is better defined in the i-band light curve (with a depth of Ai = 0.034 mag), than in the g band (Ag = 0.1 mag), in which the star is significantly fainter. Near-IR photometry obtained with the SOFI array at the ESO New Technology Telescope yields K = 12.20 ± 0.10 and V-K = 8.40 ± 0.10, so red that it is unlike any transit candidate studied before. With the new data, we consider two possible configurations for the system: (1) a nearby M7 V star or (2) a blend with a very reddened, distant red giant. The first hypothesis would give a radius for the companion of Rp = 0.3 ± 0.1 RJ, i.e., the size of Neptune. Quantitative analysis of near-IR spectroscopy finally shows that OGLE-TR-82 is a distant, reddened, metal-poor early K giant, confirmed by direct comparison with stellar templates, which gives as a best match a K3 III star. Therefore, we rule out a planetary nature for the companion, and conclude that this system is a main-sequence binary blended with a background red giant. As a case study, a system that can so mimic a planetary transit presents a lesson for future transit surveys.

Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates. II. Transits of OGLE‐TR‐113‐b in the Optical and Near‐IR

We present precise V and Ks-band transit photometry for the planetary host star OGLE-TR-113. Using the Ks-band photometry, we confirm the dwarf nature of OGLE-TR-113, and obtain new estimates for its effective temperature, distance and reddening. We employ the V-band photometry to obtain planetary and orbit parameters from the transit fit, a= (0.0232 \pm 0.0038) AU, orbital period P= (1.4324752 \pm 0.0000015) days, i= 86.7 - 90, R_p= (1.09 \pm 0.09) R_J. These values are in excellent agreement with previous works. Assuming a mass M_p= (1.32 \pm 0.19) M_J for the planet we obtain its mean density \rho= (1.26 \pm 0.50) g cm^{-3}, also in agreement with previous works. The transit observed in the Ks-band has a larger scatter and we find its amplitude to be consistent with that in the V-band. In this way, we find an independent confirmation of the planetary nature of OGLE-TR-113b.

A search for planetary-mass objects and brown dwarfs in the Upper Scorpius association

We report the results of a deep photometric search for planets and brown dwarfs in the nearby young OB Upper Scorpius association. We obtained optical (I) and near-infrared (JKs) images around nine very low-mass stars and brown dwarf member candidates of the association, covering a total area of 113 arcmin^2. Using a point spread function subtraction technique, we have searched for planetary-mass companions (0.002 Msol 0.007 Msol) at physical distances larger than 600AU. From the I, I-J colour-magnitude diagrams and follow-up Ks-band photometry, we identify four very red objects (I-J > 2.5) in the area of the survey. According to their positions in the diagram, below the expected theoretical sequence of the cluster, three are probable late M-dwarfs in the field. The faintest and reddest object (J-Ks = 2.39 +/- 0.06) may be either a field intermediate L-dwarf with anomalously red J-Ks colour at a distance of approx. 135pc, an extremely red distant galaxy, or a reddened planetary-mass object in the Upper Scorpius association.

Intermediate-age globular clusters in early-type galaxies: Better age determinations by addingU-band observations to theV,I,Kdatasets

This paper represents an extension to a series of publications on combined optical and near-infrared photometry of globular cluster systems in early-type galaxies. Based on color-color diagrams the cumulative age distributions of their globular cluster systems have been derived and revealed intermediate age clusters in two galaxies, namely NGC 4365 and NGC 5846. The extension of this observations towards the blue wavelength range was performed in order to increase the age resolution of photometric studies. In this paper we present the results of U-band observations of NGC 4365 and NGC 5846 and their combination with previously obtained V, I and Ks-band photometry. (U − I )v s. (V − Ks) color-color diagrams are used to derive the cumulative age distribution. The later is compared to simulated globular cluster systems of known age and size composition in order to set constrains on relative ages and size of the sub-populations.

Infrared Properties of Cataclysmic Variables in the 2 Micron All‐Sky Survey Second Incremental Data Release

Cataclysmic variables (CVs) have "traditionally" been observed primarily at short wavelengths because accretion-generated luminosity, which peaks in the optical-ultraviolet, dominates the radiated energy of most systems. Hence, relatively little is known about their infrared (IR) properties. Investigating CVs in the IR will contribute to the understanding of key system components that are expected to radiate at these wavelengths, such as the cool outer disk, accretion stream, and secondary star. We have compiled the near-IR J, H, and Ks band photometry of all cataclysmic variables contained in the sky coverage of the Second Incremental Data Release of the 2 Micron All Sky Survey (2MASS). This data comprises 251 cataclysmic variables with reliably identified near-IR counterparts and S/N > 10 photometry in one or more of the three near-IR bands. In addition to tables containing the 2MASS data, we present a set of near-IR finding charts for selected systems. A comparison between the 2MASS photometry and various literature sources of near-IR photometry of cataclysmic variables shows good agreement after allowing for differences in photometric systems and the intrinsic variability of cataclysmic variables. The bulk of our analysis consists of an exploration of near-IR color-color diagrams of the main cataclysmic variable classes. Results from this analysis include: (1) dwarf novae in outburst and quiescence occupy distinct regions of their color-color diagram; (2) nova-like CVs (and dwarf novae in outburst) have colors similar to F-K main-sequence stars, although this does not imply that they have F-K type secondary stars; (3) polars and intermediate polars also occupy distinct regions in their color-color diagram, with most polars having colors essentially indistinguishable from late (M0+) main-sequence stars; (4) there is no strong correlation between age and color for novae, except that many old novae (>75 yr since outburst) have colors similar to F-K main-sequence stars; and (5) there are unusual and unexplained loci of data points in all the color-color diagrams that warrant further investigation in the IR. Except in the case of the polars, near-IR photometry of cataclysmic variables does not isolate the luminosity contribution of their secondary stars. In general, the near-IR colors of cataclysmic variables are significantly and systematically offset blueward of the spectral type of secondary star expected at their orbital periods. This blue contamination of the near-IR light almost certainly originates from the accretion process. For a few systems, their near-IR color is redder than the secondary star expected at their orbital period. One effect that can explain some, but not all, of the red-excess cataclysmic variables is the presence of an evolved secondary star. We suggest that this can also be caused by the luminosity contribution of the cool outer regions of prominent accretion disks. There is at least a weak trend of redder color in higher inclination systems (in which the disk rim would be most visible and most obscure the hot inner region) that supports this hypothesis.