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\IBVShead{5105}{5 June 2001}

\IBVStitle{Time-resolved Photometry of AH E\lowercase{ri} in Outburst}

\IBVSauth{Kato, Taichi; Nogami, Daisaku$^*$}

\IBVSinsto{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
e-mail: tkato@kusastro.kyoto-u.ac.jp}

\IBVSx{Hida Observatory, Kyoto University, Gifu 506-1314, Japan,
e-mail: nogami@kwasan.kyoto-u.ac.jp}

\SIMBADobj{AH Eri}
\IBVStyp{ UGSS }
\IBVSkey{dwarf nova, photometry}

\begintext

AH Eri is a dwarf nova which had been a candidate for a system
with a short orbital period (Szkody 1987).  Szkody et al.\ (1989)
performed CCD photometry in quiescence, and found 0.1--0.3 mag
modulations with a period of $42\pm2$ min.  Szkody et al.\ (1989)
interpreted this period as the possible spin period of a magnetic
white dwarf, as in DQ Her systems.  However, since the similar period
in AL Com, which Howell and Szkody (1988) originally attributed to the
spin period, later turned out to be the double-wave modulations of the
81.6-min orbital period (for an extensive review of the object,
see Nogami et al.\ 1997), a question was raised whether the reported
42-min periodicity in AH Eri actually reflects the spin period
or is rather related to the orbital period.

The question remained unsettled until the discovery of the firm orbital
period of 5.74 hours by Thorstensen (1997).  Thorstensen (1997) also
argued against the spin-period interpretation of the $42\pm2$ min by
Szkody et al.\ (1989), based on the low strength of He II emission
lines, which are usually strong in magnetic cataclysmic variables.

An outburst of AH Eri was announced on 1997 February 28 (Hers 1997).
We performed time-resolved CCD photometry on 1997 March 1 in order to
test the presence of the claimed $42\pm2$-min periodicity.  The
observations were done using a CCD camera (Thomson TH~7882, $576 \times
384$ pixels, on-chip $2 \times 2$ binning adopted) attached to the
Cassegrain focus of the 60-cm reflector (focal length${}=4.8$~m) at
Ouda Station, Kyoto University (Ohtani et al.\ 1992).  An interference
filter was used which had been designed to reproduce the Johnson $V$
band.  The exposure time was 40 s.  The frames were first corrected for
standard de-biasing and flat fielding, and were then processed by a
microcomputer-based aperture photometry package developed by one of the
authors (TK)\@.  The magnitudes were determined relative to GSC
5319.1471 ($V=12.18$, $B-V=+0.63$), whose short-term constancy was
confirmed using GSC 5319.1526 ($V=12.56$, $B-V=+0.65$).  The magnitudes
are taken from Henden and Honeycutt (1997).  A total of 90 useful
frames were obtained.  Barycentric corrections to observed times were
applied before the following analysis.

\IBVSfig{10cm}{5105-f1.ps}{Light curve of AH Eri on 1997 March 1}

\IBVSfig{10cm}{5105-f2.ps}{Light curve of AH Eri folded by a test period of
42 min}

\vskip-22pt
The light curve drawn from these observations is presented in Figure 1.
The light curve shows a slow fading, but there were no apparent periodic
variations.  After removing the linear fading trend, we performed a period
analysis between 0\fday02 and 0\fday04 using the Phase Dispersion Minimization
(PDM) method (Stellingwerf 1978).  The analysis did not yield a significant
periodicity.  Figure 2 shows the phase-averaged light curve
folded by the reported period of 42 min.  The light curve suggests that
the 42-min period may have been marginally detected.  But because of the
lack of a firm signal in period analysis, we adopt the observed full
amplitude (0\fmm03) at the supposed 42-min period as the upper limit
of this periodicity.  The upper limit of 0\fmm03 is 3 to 10 times
smaller than reported in Szkody et al.\ (1989).  We conclude that the
claimed 42-min periodicity of AH Eri did not appear, or was markedly
reduced in amplitude, during its 1997 outburst.

The authors are grateful to VSNET members for providing observations,
and to J. Hers for promptly notifying the outburst.

\references

Henden, A. A., Honeycutt, R. K., 1997, {\it PASP}, {\bf 109}, 441

Hers, J., 1997, {\it VSNET alert circulation}, No.\ 755 (available
from\\
http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/alert0/msg00755.html)

Howell, S. B., Szkody, P., 1988, {\it PASP}, {\bf 100}, 224

Nogami, D., Kato. T., Baba, H., Matsumoto, K., Arimoto, J., Tanabe, K.,
Ishikawa, K., 1997, {\it ApJ}, {\bf 490}, 840

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru,
J., Araya, S., Ohta, K., 1992, {\it Memoirs of the Faculty of Science,
Kyoto University, Series A of Physics, Astrophysics, Geophysics and
Chemistry}, {\bf 38}, 167

Stellingwerf, R. F., 1978, {\it ApJ}, {\bf 224}, 953

Szkody, P., 1987, {\it ApJS}, {\bf 63}, 685

Szkody, P., Howell, S. B., Mateo, M., Kreidl, T. J., 1989, {\it PASP},
{\bf 101}, 899

Thorstensen, J. R., 1997, {\it PASP}, {\bf 109}, 1241

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