In several cases, eclipsing binaries are members of a close visual binary (e.g., V822 Her, Ori, V1031 Ori, V Pyx, DN UMa). Recently visual components were discovered by speckle interferometry for eclipsing binaries V 505 Sgr (McAlister et al. 1987, 1993) and SZ Cam (Mason 1995). To explain the behaviour of times of minima, presence of the light-time effect due to a third body for both systems was suggested - by Chambliss et al. (1993) for V 505 Sgr, by Mayer et al. (1994) for SZ Cam. Light-time effect might be found for some of the binaries mentioned too.
When radial velocity data have been measured for a visual binary with a known orbit, then masses of components and distance of the system can be determined. However, if the period is longer than one or two decades the spectroscopic data might not be of sufficient accuracy. In case the component is an eclipsing binary then the light-time effect can play a role identical to the spectroscopy.
V 505 Sgr (HR 7571; HD 187949) is an Algol-type system, with the primary component of spectral type A2 V, the secondary component G4 IV, and with a third body with spectral class near F8 (Tomkin 1992). Magnitude in maximum is , minimum depths are 1.2 and 0.25 mag (V), period 1.183 d. Recent radial velocity study was done by Tomkin (1992), light curves were solved e.g. by Walker (1993) and Chambliss et al. (1993).
In the following an attempt will be made to join the observed times of minima with data from speckle interferometry to find preliminary parameters of the third body orbit of the V 505 Sgr system. Interesting is probably the length of the period (connected with a forecast of the next periastron time).
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998