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\IBVShead{5316}{4 October 2002}

\IBVStitle{FT C\lowercase{am}: an Analogous Object to IR C\lowercase{om}}

\IBVSauth{Kato,~Taichi}
\vskip 5mm

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


\SIMBADobj{FT Cam}
\IBVStyp{ UG }
\IBVSkey{dwarf nova, classification}


\begintext

   FT Cam (=Antipin Var64) is a dwarf nova discovered by Antipin (1999).
The object was photometrically studied by Kato et al. (2001) during the
2000 February outburst.  Kato et al. (2001) revealed a rather monotonous
decline at a rate of 0.82 mag d$^{-1}$, without a signature of
superhumps.  From these observations and the past record, Kato et al. (2001)
suggested that FT Cam may be an SS Cyg-type star, but there remained
a possibility that we have only observed normal outbursts of an SU UMa-type
star.

   Recently, Thorstensen and Fenton (2002) reported the detection
of a spectroscopic orbital period ($P_{\rm orb}$) of 0.07492(8) d.
Such a short period (below the period gap of cataclysmic variables)
would naturally suggest an SU UMa-type classification (Warner 1995)\footnote{There are a few known dwarf novae below the period gap, which have not been yet demonstrated to be SU UMa-type stars in spite of sufficient observations.  IR Com, as we will discuss later, is such an example.}.
However, the apparent lack of long outbursts (superoutbursts) in the photometric record of FT Cam, which qualify an object to be an SU UMa-type star, has 
raised a new problem (Thorstensen and Fenton 2002).
Both Kato et al. (2001) and Thorstensen and Fenton (2002) remarked that
only short outbursts have been observed, and no superoutburst-like long
outbursts have been yet recorded.  This conclusion has been confirmed
with the observations
reported to VSNET ({\tt http://www.kusastro.kyoto-u.ac.jp/vsnet/}) up to
2002~September.  Since the object has been monitored more than 4 yr
(with occasional gaps; there remains a small possibility that some
superoutburst occurred in an unfortunate gap), the apparent lack of
superoutbursts is unusual, since the longest expected supercycle length
for SU UMa-type dwarf novae is $\sim$1000 d, unless there is a special
mechanism, such as that which seems to be working in WZ Sge-type stars
(Ichikawa and Osaki 1994).  Only three short outbursts (1998 September,
2000 February, 2002 February) have been recorded during this period.

   Among the possible ``special mechanisms" to suppress dwarf nova-type
outbursts, Kato et al. (2001) have suggested that FT Cam may be an
intermediate polar (IP), whose magnetic field can suppress dwarf nova-type
outbursts (Angelini and Verbunt 1989).  However, time-resolved photometry
by Thorstensen and Fenton (2002) did not reveal the presence of coherent
pulse, which is expected to be present in an IP.  The weakness of
He\textsc{II} emission line (Thorstensen and Fenton 2002) also prefers
the non-magnetic (non-IP) nature.

\newpage

   We alternatively propose that FT Cam is an analog of IR Com, another
peculiar dwarf nova with a short orbital period ($P$ = 0.08704 d) and
infrequent outbursts
(Kato et al. 2002 and references therein).  The number of recorded
outbursts of IR Com between 1997 and 2002 is only three, and no long
outbursts (superoutbursts) have been yet conclusively detected.  All of
these features are common to FT Cam.

\begin{table}
\begin{center}
Table 1. Comparison of X-ray Properties of FT Cam and IR Com$^a$ \\
\vspace{10pt}
\begin{tabular}{ccccc}
\hline
Object    & Count rate & HR1 & HR2 & $V$ \\
\hline
FT Cam    & 0.050 & 1.00 & 0.47 & 17.5 \\
IR Com    & 0.061 & 1.00 & 0.44 & 17.0 \\
\hline
 \multicolumn{5}{l}{{\small $^a$ The X-ray data are taken from Voges et al. (1999).}} \\
\end{tabular}
\end{center}
\end{table}

   We also note that the X-ray properties of FT Cam is extremely close
to those of IR Com (Table 1).  In particular, the remarkable agreement
in hardness ratios and flux ratios (X-ray count rate/optical flux)
is striking.  In view of these properties, as well as remarkably similar
outburst properties, FT Cam and IR Com make almost a ``twin" among
short-$P_{\rm orb}$ dwarf novae.  Up to now, HT Cas has been proposed to
have analogous properties with IR Com (Kato et al. 2002).  Since both
IR Com and HT Cas are eclipsing systems, the presence of a non-eclipser
FT Cam provides us new opportunities in studying these unusual systems
at different binary inclinations.  Since HT Cas is known to very
infrequently show superoutbursts (e.g. Zhang et al. 1986; no superoutburst
has been recorded since 1985), we still have chance to eventually see
a superoutburst of FT Cam.  Future confirmation of such a superoutburst
will provide an observational test for proposed mechanisms of suppressing
outbursts in some unusual short-$P_{\rm orb}$ dwarf novae
(e.g. Lasota et al. 1995).

\vskip 3mm

We are grateful to all observers who reported vital observations to VSNET.
This work is partly supported by a grant-in aid (13640239) from the
Japanese Ministry of Education, Culture, Sports, Science and Technology.

\references

Angelini, L., \& Verbunt, F., 1989, {\it MNRAS}, {\bf 238}, 697

Antipin, S. V., 1999, {\it IBVS}, 4673

Ichikawa, S., \& Osaki, Y., Duschl, W. J. et al., 1994, {\it in Theory of Accretion Disks-2}, (Dordrecht: Kluwer Academic Publishers), {\bf p. 169}

Kato, T., Uemura, M., \& Yamaoka, H., 2001, {\it IBVS}, 5082

Kato, T., Baba, H., \& Nogami, D., 2002, {\it PASJ}, {\bf 54}, 79

Lasota, J.-P., Hameury, J.-M., \& Hur\'{e}, J. M., 1995, {\it A\&A}, {\bf 302}, L29

Thorstensen, J. R., \& Fenton, W. H., 2002, {\it PASP}, in press
   (astro-ph/0209172)

Voges, W., Aschenbach, B., Boller, T. et al., 1999, {\it A\&A}, {\bf 349}, 389

Warner, B, 1995, {\it ApSS}, {\bf 226}, 187

Zhang, E.-H., Robinson, E. L.,\& Nather, R. E., 1986, ApJ, {\bf 305}, 740

\endreferences

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