THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS
25 Birch Street, Cambridge, MA 02138 USA
INTERNET: aavso@aavso.org
Tel. 617-354-0484     FAX 617-354-0665

AAVSO PHOTOELECTRIC PHOTOMETRY ALERT NOTICE

22 VULPECULAE CAMPAIGN

The composite-spectrum star 22 Vul is a binary system consisting of a
luminous mid-G giant primary and a relatively faint late-B dwarf secondary.
The orbital period is 249 days.  The secondary component was detected in
1983 by the International Ultraviolet Explorer satellite, but an observation
on April 17, 1983, found the secondary spectrum absent and the total light
fainter than usual, indicating eclipses.

Additional IUE spectra revealed that, before and after the eclipses, 22 Vul
shows the strong absorption spectrum characteristic of the
atmospheric-eclipse phenomenon in zeta Aur systems.  These systems have a
special astrophysical significance: when the system is near eclipse, the
line of sight to the hot dwarf passes through various layers of the cool
giant's chromosphere, causing phase-dependent absorption lines to appear.
The strength of these lines yields information about the temperature and
density of the chromospheric layers in the line of sight.

In 1988 the system was studied in the optical, both photometrically and
spectroscopically.  Phase-dependent chromospheric absorption lines and the
eclipse light-curves were measured and analyzed, yielding the geometry of
the system and a model of the density gradient in the chromospheric layers.
Spectroscopic observations were, however, limited to ingress only.

The next mid-eclipse of 22 Vul will occur on September 2.2 UT.  Astronomer
Elizabeth Griffin hopes to obtain more extensive spectroscopic observations,
from the extreme ingress to extreme egress, and has asked that the AAVSO's
photoelectric photometrists support this project by contributing UBV
observations.

Since the hot secondary is the component being eclipsed, and because the
material in the giant's chromosphere is strongly absorbing in the near UV,
the duration and depth of minimum decreases with increasing wavelength.
Totality lasted about 8.9 days in 1988, with photometric decreases of DU =
0.35 magnitude, DB = 0.13, DV = 0.05.  Since the minimum is so shallow in V,
observations are particularly needed in the B and especially U passbands as
well as V.  Since the partial eclipse phases last more than one day,
observers are needed at longitudes around the world in order to obtain the
necessary coverage.  All observers should use the same comparison and check
stars:

			   V		 B-V		 U-B
Variable	22 Vul		+5.15		+1.04       +0.71
Comparison	24 Vul		+5.32		+0.95		+0.67
Check		18 Sge		+6.13		+1.04		+0.92


22 Vul = RA (2000) 20 15 30.1, Dec (2000) +23 30 31
HR 7741
HD 192713

PEP Alert Notice - 2


24 Vul = RA (2000) 20 16 47.0, Dec (2000) +24 40 16
HR 7753
HD 192944

18 Sge = RA (2000) 20 16 19.6, Dec (2000) +21 35 55 
HR 7746
HD 192836


Useful observations can be made throughout the interval from August 20 to
September 15.  The duration of the partial eclipse intervals (first to
second contact and third to fourth contact) varies from about 1.2 days in V
to 1.7 days in B to 2.7 days in U.  Coverage of the extreme shoulders of the
light curves at first and fourth contacts is particularly needed.  Before
and after the eclipse and during totality, one set of observations per night
is adequate.  During the intervals of partial eclipse, August 25-29 and
September 6-10, a second set of observations later in the evening might
prove useful.

Each observation should include at least three differential measures of the
variable in each filter and an equal number of measures of the check star.
The observations must be corrected for differential extinction and, because
data from several observers will be combined, observations must be fully
transformed to standard Johnson differential magnitudes.  It is important
for each observer to strive for the highest possible precision.

The preferred reporting format is Heliocentric JD to three decimal places,
the number of differential measures included in each observation, the mean
differential magnitude in each filter for the variable and check star to
three decimal places, and the standard deviation.  Alternatively, report the
HJD and the individual differential magnitudes.

David Williams, a photoelectric observer and member of the AAVSO Eclipsing
Binary Committee, will collect the observations, check quality, and forward
the data for analysis and archiving.  Send observations to: David B.
Williams, P. O. Box 58, Whitestown, IN 46075 U.S.A., telephone 317-769-7430,
e-mail dbwilyumz@aol.com.

We encourage all of our photoelectric observers to contribute to this
special project.  Good observing!

Janet A. Mattei
Director

David B. Williams
Editor, Eclipsing Binary Update