-We have monitored 41 Be/X-ray binary systems in the Small Magellanic Cloud over ∼ 9 years using PCA-RXTE data from a weekly survey program. The resulting light curves were analysed in search of orbital modulations with the result that 10 known orbital ephemerides were confirmed and refined, while 10 new ones where determined. A large number of X-ray orbital profiles are presented for the first time, showing similar characteristics over a wide range of orbital periods. Lastly, three pulsars: SXP46.4, SXP89.0 and SXP165 were found to be misidentifications of SXP46.6, SXP91.1 and SXP169, respectively. Subject headings: galaxies: individual (Small Magellanic Cloud) -pulsars: general -X-rays: binaries Fig. 6.-SXP6.85. a) Top: X-ray amplitude light curve. b) Middle: Lomb-Scargle power

We present results of RXTE observations of the low-mass X-ray binary and atoll source 4U 1608−52 made over 9 days during the decline of an X-ray intensity outburst in March 1996. A fast-timing analysis shows a strong and narrow quasi periodic oscillation (QPO) peak at frequencies between 850 and 890 Hz on March 3 and 6, and a broad peak around 690 Hz on March 9.Observations on March 12 show no significant signal. On March 3, the X-ray spectrum of the QPO is quite hard; its strength increases steadily from 5 % at ∼2 to ∼20 % at ∼12 keV. The QPO frequency varies between 850 and 890 Hz on that day, and the peak widens and its rms decreases with centroid frequency in a way very similar to the well-known horizontal branch oscillations (HBO) in Z-sources. We apply the HBO beat frequency model to atoll sources, and suggest that, whereas the model could produce QPOs at the observed frequencies, the lack of correlation we observe between QPO properties and X-ray count rate is hard to reconcile with this model. Subject headings: stars: individual (4U 1608−52) -stars: neutron -accretion, accretion disks However, in spite of these arguments, the absence of a correlation between X-ray intensity on the one hand and QPO frequency and rms amplitude on the other hand poses a major problem for a beat frequency model interpretation. The QPO frequency remains constant near 850 Hz between 3 and 6 March while the count rate drops by more than a factor 4, from ∼3200 to ∼600 counts/sec (Sect. 2). The beat frequency model predicts that if the QPO frequency remains constant, the mass flow through the inner edge of the disk should remain constant as well. Our data therefore are inconsistent with a beat frequency model interpretation if all accretion takes place via the inner edge of the disk. We note that the presence of a hypothetical additional (non-disk) mass flow component contributing more than 75% of the total flux on March 3 and much weaker on March 6 can not easily resolve this discrepancy. The rms amplitude of the QPO only increases by a factor 2 from March 3 to 6 while the X-ray intensity drops by a factor of more than 4, whereas a similar fractional change would be predicted in this explanation. Only by invoking rather large and entirely ad-hoc changes in the beaming or bolometric correction could the model be maintained. The 150 Hz drop in QPO from March 6 to March 9 without a change in count rate presents similar difficulties. Further observations of 4U 1608−52 during other outbursts, and study of its X-ray bursts are required to shed further light on the relation of the 800 Hz QPO in 4U 1608−52 with those in 4U 1728−34, and with the QPO in Sco X-1. Sztajno M. 1985, Nature 316, 225 van der Klis M., 1989. NATO ASI C262: Timing Neutron Stars,Ögelman and van den Heuvel (eds.), Kluwer, p. 27. van der Klis M. 1995, in X-ray Binaries, Van Paradijs and Van den Heuvel (eds.), Cambridge University Press, p. 252.

Results of a 4 year X-ray monitoring campaign of the Small Magellanic Cloud using the Rossi X-ray Timing Explorer (RXTE) are presented. This large dataset makes possible detailed investigation of a significant sample of SMC X-ray binaries. 8 new X-ray pulsars were discovered and a total of 20 different systems were detected. Spectral and timing parameters were obtained for 18. In the case of 10 pulsars, repeated outbursts were observed, allowing determination of candidate orbital periods for these systems. We also discuss the spatial, and pulse-period distributions of the SMC pulsars.