Late-type near-contact eclipsing binary [HH97] FS AUR-79

The secondary photometric standard star number 79 for the FS Aur field (Henden & Honeycutt 1997), designated as [HH97] FS Aur-79 (GSC 1874-399), is a short-period (0.2508 days) eclipsing binary whose light curve is a combination of the βLyr and BY Dra type variables. High signal-to-noise ratio multicolor photometry was obtained using the US Naval Observatory 1 m telescope. These light curves show asymmetry at quadrature phases (the O'Connell effect), which can be modeled with the presence of starspots. A low-resolution spectrum obtained with the 3.5 m Wisconsin-Indiana-Yale-NOAO telescope at orbital phase 0.76 is consistent with a spectral type of dK7e and dM3e. A radial velocity curve for the primary star was constructed using 24 high-resolution spectra from the 9.2 m Hobby-Eberly Telescope. Spectra show Hα and Hβ in emission confirming chromospheric activity and possibly the presence of circumstellar material. Binary star models that simultaneously fit the U, B, V, R, and radial velocity curves are those with a primary star of mass 0.59 ±0.02 M_⊙, temperature 4100 ±25 K, and mean radius 0.67 R_⊙, just filling its Roche lobe, and a secondary star of mass 0.31 ±0.09 M_⊙, temperature 3425 ±25 K, and mean radius 0.48 R_⊙, just within its Roche lobe. An inclination angle of 83° ±2° with a center-of-mass separation of 1.62 R _⊙ is also derived. Starspots, expected for a rotation period of less than 1 day, had to be included in the modeling to fit the O'Connell effect.