  We report the discovery of a new class of low frequency quasi--periodic
variations 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 mHz. While usually the relative amplitude of flux variations
increases with energy, the newly discovered QPOs are limited to the softest
energies (1-5 keV). The observations of 4U1608-52 suggest that 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 exist.
  Judging from this complex of properties, we speculate that a special mode of
nuclear burning at the neutron star surface is responsible for the observed
flux variations. Alternatively, some instabilities in the accretion disk may be
responsible for these QPOs.
