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We present new photometric observations in the BVRI bands of the double-lined eclipsing binary BD +05706 conducted over three observing seasons, as well as new X-ray observations obtained with ROSAT covering a full orbital cycle (P = 18.9 days). A detailed light-curve analysis of the optical data shows the system to be semidetached, confirming indications from an earlier analysis by Torres et al. (published in 1998), with the less massive and cooler star filling its Roche lobe. The system is a member of the rare class of cool Algol systems, which are different 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 new photometry with a reanalysis of the spectroscopic observations reported by Torres et al., we derive accurate absolute masses for the components of M1 = 2.633 + 0.028 M and M2 = 0.5412 + 0.0093 M, radii of R1 = 7.55 + 0.20 R and R2 = 11.02 + 0.21 R, as well as effective temperatures of 5000 + 100 and 4640 + 150 K, for the primary and secondary, respectively. There are obvious signs of activity (spottedness) in the optical light curve of the binary. Our X-ray light curve clearly shows the primary eclipse but not the secondary eclipse, suggesting that the primary star is the dominant source of the activity in the system. The depth and duration of the eclipse allow us to infer some of the properties of the X-ray–emitting region around that star.



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