New class of low frequency QPOs: Signature of nuclear burning or accretiondisk instabilities?

We report the discovery of a new class of low frequency quasi-periodic vari ations of the X-ray flux in the X-ray bursters 4U1608-52 and 4U1636-536. We also report an occasional detection of a similar QPO in Aql X-1. The QPOs, associated with flux variations at the level of percents, are observed at a frequency of 7-9 x 10(-3) Hz. While usually the relative amplitude of flu x variations increases with energy, the newly discovered QPOs are limited t o the softest energies (1-5 keV). The observations of 4U1608-52 suggest tha t these QPOs are present only when the source X-ray luminosity is within a rather narrow range and they disappear after X-ray bursts. Approximately at the same level of the source luminosity, type I X-ray bursts cease to exis t. Judging from this complex of properties, we speculate that a special mod e of nuclear burning at the neutron star surface is responsible for the obs erved flux variations. Alternatively, some instabilities in the accretion d isk may be responsible for these QPOs.