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IAUC 2794: 3U 0900-40; Cyg X-3; HD 87643

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                                                  Circular No. 2794
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.

3U 0900-40
     S. Rappaport and J. McClintock, Massachusetts Institute of
Technology, report that the SAS-3 group has discovered a 282s.9 +/-
0s.4 periodicity in the x-ray intensity of the x-ray/optical binary
system 3U 0900-40/HD 71581.  The source was detected between June
18.8 and 24.5 UT in the energy range 1.5-30 keV.  The light curve
has an asymmetric double pulsed structure with at least a 60 percent
modulation.  The source exhibited an extremely hard spectrum
and a high degree of irregular variability, on time scales from
seconds to hours, in addition to the periodic phenomenon.  The
identification is based on a positional coincidence within an
uncertainty area of 0.15 square degrees (90-percent confidence) and
the absence of any signal during a predicted eclipse of 3U 0900-40.
The available data are not yet sufficient to define the Doppler
effect due to orbital motion.

     E. Kendz Orra, W. Pietsch, R. Staubert and J. Trumper,
Astronomisches Institut, Tubingen, report: "Cyg X-3 was observed on Feb.
20.64 UT using a balloon borne hard x-ray telescope launched from
Palestine, Texas.  In the energy range 29-70 keV an intensity
variation was detected that is in phase with the low energy 4h.8
sinusoidal variation.  The relative amplitude at 29-70 keV is 0.37 +/-
0.19.  Comparing this value with the results of Uhuru (2-6 keV) and
OSO-7 (7-10 keV), there is no indication for an energy dependence
of the relative amplitude."

HD 87643
     W. Wamsteker, European Southern Observatory, reports: "Recent
observations of the star HD 87643 (R.A. = 10h02m49s, Decl. = -58o25'.4,
equinox 1950.0; spectral type Be) give a visual magnitude V = 7.40.
This is one magnitude brighter than in 1968 (Hiltner et al. 1968,
Astrophys. Lett. 2, 153).  The infrared flux also seems to have
changed by Delta-(K - L) = +0.72 compared with Allen (1973, Monthly
Notices Roy. Astron. Soc. 161, 145).  Spectrum scans indicate an
increase in expansion velocity.  This star might be in an active
phase as a slow nova, possibly representing the formation of an eta
Car-type object.  Observations of this object in any mode could
therefore be of great interest."

1975 June 30                   (2794)              Brian G. Marsden

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