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 their X-ray "off" states. The first observations of X1755-338 in quiescence show that the counterpart, V4134 Sgr, has faded by more than 3.5 mag in V. If the mass donor in the system is an M0V star as implied by the period, our upper limits on the brightness of the counterpart suggest that it is more distant than 4 kpc. We observed V2134 Oph, the optical counterpart of X1658-298, on several occasions in April/May 1997 and found the source only about 1 mag fainter than when it is X-ray bright. Contemporaneous X-ray data confirm that the source remains in the quiescent state during our optical observations. Our optical lightcurve, folded on the 7.1 hour orbital period, does not show any modulation across the binary cycle. It is possible that the absence of detectable X-ray emission, despite the indication for an accretion disk and activity in the system, is related to structure in the disk that permanently obscures the central X-ray source. The optical properties of V2134 Oph are unique among the known X-ray transients.