  The type I X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was
monitored by the Rossi X-ray Timing Explorer between February and October,
1996. The source was observed in various spectral states; the highest
luminosity state (LX ~ 1.5 10^37 ergs/s, 10 kpc, 1-20 keV) is associated with a
soft spectrum, whereas for the lower luminosity state (down to ~5 10^36 ergs/s)
the spectrum is significantly harder. Using the high time resolution data
provided by the Proportional Counter Array (PCA), we have discovered
High-Frequency Quasi-Periodic Oscillations (HFQPOs) in the persistent X-ray
emission of 4U1915-05 while its luminosity was ~8 10^36 ergs/s. The QPO
frequency ranges from 600 Hz up to ~1000 Hz, with typical Full Width at Half
Maximum (FWHM) ~50-100 Hz and Root Mean Squared (RMS) values 15%. In addition,
by using the ``shift and add'' technique, we have detected a twin HFQPO (5.5
sigma level) separated from the main peak by ~355 Hz. 4U1915-05 is the eighth
Atoll source displaying simultaneous twin HFQPOs. Based on current knowledge of
HFQPO sources, our observations suggest that 4U1915-05 might contain a 2.8 (or
5.6) millisecond rotating neutron star.
