Optical and X-ray observations of 4U2129+47/V1727 CYG in a quiescent state

The 5.2 h X-ray binary 4U2129+47 was observed for more than 1 orbital cycle on 29 Septemter and 4 October 1983 using the LE, Me and GSPC detectors of EXOSAT. In neither detector is an X-ray flux from the source detected. Quasi-simultaneous UBV observations using a 2.2 m telescope failed to detect the large amplitude light curve reported in earlier observations but show the optical companion in a low intensity state. The large amplitude light curves is interpreted as due to X-ray heating of the optical star by the X-ray source similar to the system Her X-1/Hz Her. Optical observations indicate that the heating X-ray source is shut off and explain that EXOSAT failed to detect the source; 4U2129+47 behaves similar to Her X-1 in the optical quiescent state.