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\begin{document}

\IBVShead{5099}{30 May 2001}

\IBVStitle{Unusual Slow Fading of Standstill in AT C\lowercase{nc}}

\IBVSauth{Kato, Taichi$^1$; Stubbings, Rod$^2$; Reszelski, Maciej$^3$; Muyllaert, Eddy$^4$;\\ \indent Simonsen, Mike$^5$; Poyner, Gary$^6$; Dubovsky, Pavol A.$^7$; Pearce, Andrew$^8$;\\ \indent Kinnunen, Timo$^9$; Maehara, Hiroyuki$^{10}$}

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

\IBVSinst{19 Greenland Drive, Drouin 3818, Victoria, Australia, e-mail: stubbo@qedsystems.com.au}

\IBVSinst{Al.\ 1-go Maja 29/4, 64500 Szamotuly, Poland, e-mail: macres@pro.onet.pl}

\IBVSinst{VVS Belgium --- Werkgroep Veranderlijke Sterren, Eksterstraat
6, 8400 Oostende, Belgium,\\ \indent e-mail: eddy.muyllaert@ping.be}

\IBVSinst{46394 Roanne Drive Macomb, MI, USA 48044, e-mail: mikesimonsen@mindspring.com}

\IBVSinst{BAA Variable Star Section, 67 Ellerton Road, Kingstanding,
Birmingham B44 0QE, England,\\ \indent e-mail: gp@star.sr.bham.ac.uk}

\IBVSinst{MEDUZA group, Vedecko-kulturne centrum na Orave, 027 42
Podbiel 194, Slovakia, e-mail: vkco@isternet.sk}

\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia, e-mail: Andrew.Pearce@worley.com.au}

\IBVSinst{Sinirinnantie 16, SF-02660 Espoo, Finland, e-mail: stars@personal.eunet.fi}

\IBVSinst{Variable Star Observers League in Japan (VSOLJ), Namiki
1-13-4, Kawaguchi, Saitama 332-0034, Japan,\\ \indent e-mail: mira@cablenet.ne.jp}


\SIMBADobj{AT Cnc}
\IBVStyp{ UGZ }
\IBVSkey{dwarf novae - photometry}

\begintext

AT Cnc is a well-established Z Cam-type dwarf nova with clear
alternations of dwarf nova-type phase and standstills (see Nogami et
al.\ 1999 for the extensive review on this object).  The presence of
standstills, in addition to outbursts, is the defining feature of Z Cam
stars, and standstills are considered to be equivalent to
non-outbursting novalike (NL) variables with thermally stable accretion
disks (cf.\ Osaki 1996 for a review of disk instability in dwarf
novae).  Standstills in Z Cam stars usually terminate with a sudden
fading to quiescence, whose rate of decline is roughly equal to the
rate of decline from outbursts.

\IBVSfig{10cm}{5099-f1.ps}{Light curve of AT Cnc.  The solid line shows
the decline at the rate of 0.0054 mag\,d$^{-1}$}

\IBVSfig{10cm}{5099-f2.ps}{Light curve of AT Cnc in 1998, showing
normal outburst cycles}

\vskip-22pt The recent standstill of AT Cnc began in 2000 November, and
has shown peculiar phenomena until now, 2001 May.  [N.B\@. The portion
prior to the standstill was not well covered by observations, since the
object was only visible in the morning twilight.  Several negative
estimates with upper limits below 14th magnitudes indicate that the
object had not started this standstill before October.] The object has
been monitored as a part of VSNET Collaboration
(http://www.kusastro.kyoto-u.ac.jp/vsnet/).  Visual observations have
been made by a number of observers, using the comparison star sequences
calibrated in the $V$ band.  The typical error of visual estimates is
0\fmm2 mag, which will not affect the following discussion.  The light
curve drawn from these observations is presented in Figure~1.  A
portion of the long-term light curve of AT Cnc is presented for
comparison in Figure~2, which clearly shows the normal outbursting
state of this dwarf nova.  The remarkable difference between the
figures demonstrates how unusual the present behavior is.  The
long-term light curve covering the 1995--2001 period is also available
at http://www.kusastro.kyoto-u.ac.jp/vsnet/LCs/index/CNCAT.html . The
first noticeable event during this standstill occurred in 2000 December
(around JD 2451900), an outburst starting from the standstill.  Since
dwarf nova outbursts are believed to be a consequence of thermal disk
instability, which occurs when the mass-transfer rate is below a
certain limit, $\dot{M}_{\rm crit}$ (cf.\ Osaki 1996), such an outburst
from the standstill is unexpected if it is triggered by the thermal
disk instability.  Another small outburst occurred in 2001 April
(around JD 2452026), but this may be of different nature, as it was on
the slow fading branch on the standstill, as described below.

The other very peculiar feature of this standstill is the slow fading
throughout the standstill (except two ``outbursts'' described above).
Such a trend is extremely peculiar among all known Z Cam stars.  The
rate of decline, measured by linear fitting to the light curve, after
removing the two ``outbursts'', is 0.0054 mag\,d$^{-1}$, corresponding
to the $e$-folding time of $\sim 200$~d, which is totally inconsistent
with the usual fading rate (0.3--0.4 mag\,d$^{-1}$) of this dwarf
nova.  This fading rate is more characteristic to slow fadings in VY
Scl-type novalike variables (cf.\ Table 1 of Honeycutt et al.\ 1994).
Together with the similarity of standstills with novalike variables,
this similarity of fading rate and pattern suggests that we may be
witnessing a VY Scl-type phenomenon (temporary reduction of
mass-transfer rate) in a Z Cam star.  However, as discussed in
Honeycutt et al.\ (1994) and also in King et al.\ (1998), the standard
disk-instability theory predicts that the system should undergo dwarf
nova outbursts as the mass-transfer rate decreases.  What has been
observed in AT Cnc is the contrary:  the ``outbursts'' are much more
infrequent and smaller than in its normally outbursting state.

In VY Scl-type stars, an idea has been proposed to solve the same
dilemma (Leach et al.\ 1999).  Leach et al.\ (1999) could reproduce the
VY Scl-type fading, without causing major outbursts, by taking the
irradiation by the hot white dwarf into account.  One may speculate
that the same process may be taking place in the present peculiar
standstill of AT Cnc.  This is not the only similarity of AT Cnc with
VY Scl-type stars.  Nogami et al.\ (1999) discovered an intermittent P
Cyg-type absorption feature which they interpreted as winds.  Such
intermittent winds are more commonly seen in VY Scl-type stars, and are
rare in dwarf novae.  The best example is BZ Cam (originally discovered
by J. R. Thorstensen and presented in Patterson et al.\ 1996; see also
Ringwald and Naylor 1998).  The spectroscopic evidence of similarity of
AT Cnc to BZ Cam was already addressed by Nogami et al.\ (1999), and
the present observation of standstill may be an additional support to
the relation between these seemingly different classes of objects.
There may be a common underlying mechanism to produce the observed VY
Scl-type or VY Scl-like features, as well as high-speed winds, in AT
Cnc and BZ Cam.  Since some of VY Scl-type stars are suspected to be
steadily burning hydrogen on the surface of their white dwarfs (Greiner
et al.\ 1999), the present anomalous state of AT Cnc would be an
attractive target to search for similar phenomenon in a Z Cam-type
dwarf nova.

\references

Greiner, J., Tovmassian, G. H., di Stefano, R., Prestwich, A.,
Gonzalez-Riestra, R., Szentasko, L., Chavarra, C., 1999, {\it A\&A},
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Honeycutt, R. K., Cannizzo, J. K., Robertson, J. W., 1994, {\it ApJ},
{\bf 425}, 835

King, A. R., Cannizzo, J. K., 1998, {\it ApJ}, {\bf 499}, 348

Leach, R., Hessman, F. V., King, A. R., Stehle, R., Mattei, J., 1999,
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Nogami, D., Masuda, S., Kato, T., Hirata, R., 1999, {\it PASJ}, {\bf
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Osaki, Y. 1996, {\it PASP}, {\bf 108}, 39

Patterson, J., Patino, R., Thorstensen, J. R., Harvey, D., Skillman, D.
R., Ringwald, F. A., 1996, {\it AJ}, {\bf 111}, 2422

Ringwald, F. A., Naylor, T., 1998, {\it AJ}, {\bf 115}, 286

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