Optical Photometry and Spectroscopy of the Suspected "Cool Algol" AV Delphini: Determination of the Physical Properties

We present new spectroscopic and BVRI photometric observations of the double-lined eclipsing binary AV Del (period ¼ 3:85 days) conducted over six observing seasons. A detailed radial velocity and light-curve analysis of the optical data shows the system to be most likely semidetached, with the less massive and cooler star filling its Roche lobe. The system is probably a member of the rare class of ''cool Algol'' systems, which are distinguished from the ''classical'' Algol systems in that the mass-gaining component is also a late-type star rather than a B- or A-type star. By combining the spectroscopic and photometric analyses, we derive accurate absolute masses for the components of M1 ¼ 1:453 0:028 Mand M2 ¼ 0:705 0:014 Mand radii of R1 ¼ 2:632 0:030 Rand R2 ¼ 4:233 0:060 R , as well as effective temperatures of 6000 200 and 4275 150 K for the primary and secondary, respectively. There are no obvious signs of activity (spottedness) in the optical light curve of the binary.