PHOTOMETRIC PROPERTIES FOR SELECTED ALGOL-TYPE BINARIES. IV. AV HYDRAE AND DZ CASSIOPEIAE

Abstract

We present BVR photometric observations and several eclipsing times for AV Hya and DZ Cas from 2004 to 2011. Using the Wilson-Devinney method, the photometric solutions with hot spots were deduced from their asymmetric light curves. The results indicate that both stars are Algol-type binaries, whose mass ratio, q(ph), and fill-out factor of the primary, f(1), are q(ph) = 0.255(+/- 0.002) and f(1) = 81.2(+/- 0.2)% for AV Hya, and q(ph) = 0.093(+/- 0.003) and f(1) = 98.7(+/- 0.3)% for DZ Cas. Based on all available light minimum times, it is discovered that the O-C curve of each star could be described by a light-time orbit overlying on a downward parabola. Their periods and amplitudes are P-3 = 37.2(+/- 0.7) yr and A = 0.(d)0095(+/- 0.(d)0006) for AV Hya, and P-3 = 62.5(+/- 1.0) yr andA = 0.(d)0183(+/- 0.(d)0007) for DZ Cas. Cyclic variations may result from the light-time effect due to the third body. The secular period decrease rates are dP/dt = -1.47(+/- 0.04) x 10(-7) days yr(-1) for AV Hya and dP/dt = -0.92(+/- 0.04) x 10(-7) days yr(-1) for DZ Cas. This may be interpreted using mass and angular momentum loss from the system. With decreasing period, the fill-out factor of the primary increases and it may finally fill its inner Roche lobe. Therefore, AV Hya and DZ Cas with a secular period decrease will evolve from semi-detached configurations into contact ones.