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IAUC 6475: P/1996 S1; NGC 6624

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IAUC number

                                                  Circular No. 6475
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444 (for emergency use only)

     J. V. Scotti, University of Arizona, reports his recovery of
comet P/1990 R2 (= 1990k = 1990 XVIII) with the Spacewatch
telescope at Kitt Peak.  The comet appears essentially stellar with
a slight hint of diffuseness, and is close to the prediction on MPC
23482 (ephemeris on MPC 26588).

     1996 UT             R.A. (2000) Decl.        m1
     Sept.19.20204   21 39 56.19   -29 50 18.2   20.2
          19.21964   21 39 55.54   -29 50 10.1   20.1
          19.23678   21 39 54.95   -29 50 02.9   20.1
          20.20399   21 39 23.78   -29 42 54.9   20.6
          20.22183   21 39 23.17   -29 42 48.2   20.5
          20.23900   21 39 22.61   -29 42 39.2   20.4

NGC 6624
     S. F. Anderson, B. Margon, and E. W. Deutsch, University of
Washington; R. A. Downes, Space Telescope Science Institute; and R.
G. Allen, University of Arizona, report:  "A Hubble Space Telescope
observation of the ultraviolet-bright counterpart to the 11-min
binary x-ray source in the core of the globular cluster NGC 6624
(King et al. 1993, Ap.J. 413, L117) shows a highly-statistically
significant ultraviolet flux modulation with an 11.5-min (+/- 0.2
min) period, consistent with the 685-s period of the known x-ray
modulation.  The ultraviolet amplitude is very large compared with
the observed x-ray oscillations:  x-ray variations are generally
reported as 2-3 percent peak-to-peak, whereas our data show an
amplitude of order 17 +/- 4 percent in the range 125-250 nm.  These
Faint Object Spectrograph data also have spectral resolution of 0.7
nm, so later analysis may reveal spectral-dependent fluctuations in
this amplitude.  Arons and King (1993, Ap.J. 413, L121) have
predicted periodic ultraviolet fluctuations in this shortest-known-
period binary system, due to the cyclically changing aspect of the
x-ray-heated face of the secondary star.  However, this predicted
modulation has never before been observed.  Employing their
formalism, which invokes a number of different physical assumptions,
we infer a system orbital inclination between about 35 and 55 deg.
Additional observations of this exotic system are badly needed."

                      (C) Copyright 1996 CBAT
1996 September 20              (6475)            Daniel W. E. Green

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