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1 Introduction

The variable FG Hydrae (BD +03$^\circ$ 1979) was discovered by Hoffmeister (1934). It was classified as a cluster type by Tsesevich (1949) by using visual observations. The first photoelectric photometric observations were obtained by Smith (1955, 1963), who discovered that the system belonged to an W Ursae Majoris type that undergoes complete eclipses. The complete light curves of the system were obtained by Binnendijk (1963), Yang et al. (1991) and Mahdy et al. (1985). According to Qian et al. (1999), the orbital period of FG Hya underwent several sudden jumps in 1952, 1962, 1982, 1985 and 1989 and the light curves of the system also show the variable O'Connell effect. The photometric solutions were obtained by Mochnacki et al. (1972), Twigg (1979), Lafta et al. (1986), Jabbar et al. (1986) and Yang et al. (1991). Their studies indicate that the system is an A-type over-contact W UMa binary. Recently Wenxian Lu & Slavek M. Rucinski (1999) published radial velocity studies for this binary. The external error of the derived radial velocities from their DDO observations is smaller than 1.8 km s-1. The derived mass ratio from their better phase covered radial velocity curve is 0.112(04) which is close to those from the photometric solutions presented by Mochnacki et al. (1972), Twigg (1979), Lafta et al. (1986), Jabbar et al. (1986) and Yang et al. (1991).

Because of the variable O'Connell effect seen in the light curve and the over-contact nature of the binary, FG Hya was included in the observing program of short period variables at the Yunnan Observatory using the 100-cm reflector telescope.


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