Optical Studies of the Transient Dipping X-Ray Sources X1755−338 and X1658−298 in Quiescence

We have studied the optical counterparts of X1755-338 and X1658-298 in theirX-ray "off" states. The first observations of X1755-338 in quiescence show thatthe counterpart, V4134 Sgr, has faded by more than 3.5 mag in V. If the massdonor in the system is an M0V star as implied by the period, our upper limitson the brightness of the counterpart suggest that it is more distant than 4kpc. We observed V2134 Oph, the optical counterpart of X1658-298, on severaloccasions in April/May 1997 and found the source only about 1 mag fainter thanwhen it is X-ray bright. Contemporaneous X-ray data confirm that the sourceremains in the quiescent state during our optical observations. Our opticallightcurve, folded on the 7.1 hour orbital period, does not show any modulationacross the binary cycle. It is possible that the absence of detectable X-rayemission, despite the indication for an accretion disk and activity in thesystem, is related to structure in the disk that permanently obscures thecentral X-ray source. The optical properties of V2134 Oph are unique among theknown X-ray transients.Comment: 8 pages including 3 figures and 1 table; Uses AASTeX 4.0; Accepted for publication in The Astrophysical Journal Letter