A variable 0.58-2.44 Hz quasi-periodic oscillation in the eclipsing and dipping low-mass X-ray binary EXO 0748-676

We report the discovery of a quasi-periodic oscillation (QPO) in data obtai ned with the Rossi X-Ray Timing Explorer of the dipping and eclipsing low-m ass X-ray binary EXO 0748-676. The QPO had a frequency between 0.58 and 2.4 4 Hz changing on timescales of a few days and an rms amplitude between 8% a nd 12%, and it was detected in the persistent emission during dips and duri ng type I X-ray bursts. During one observation, when the count rate was a f actor of 2-3 higher than otherwise, the QPO was not detected. The strength of the QPO did not significantly depend on photon energy and is consistent with being the same in the persistent emission, both during and outside the dips and during type I X-ray bursts. Frequency shifts were observed during three of the four X-ray bursts. We argue that the QPO is produced by the s ame mechanism as the QPO recently found by Jonker et al. in 4U 1323-62. Alt hough the exact mechanism is not clear, it is most likely related to the hi gh inclination of both systems. An orbiting structure in the accretion disk that modulates the radiation from the central sourer seems the most promis ing mechanism.