3 Notes on individual objects

3.1 AL Com

AL Com was discovered and classified as an eruptive variable star by Rosino (1961). At quiescence it is very faint ($m_{\rm pg} \simeq 20$), but large amplitude ($\simeq 8$ mag) outbursts have been detected in 1961, 1965, 1974 and 1975 (Bertola 1964; Moorehead 1965; Scovil 1975).

Recently, Patterson et al. (1996) and Kato et al. (1996) have suggested a period of 81.6 min as the orbital period of the binary system. There are also brightness variations in quiescence at a period which varies between 80 and 90 min (see, e.g., Howell & Szkody 1991; Abbott et al. 1992).

We intermittently monitored AL Com since 1994. It was always below the limit of detectability of our instrument, until the beginning of April 1995 when, after about 20 years of quiescence, it started to increase its brightness. The light curve during this rare superoutburst has been well sampled by the American Association of Variable Star Observers (AAVSO), by the Astronomer Group and by many others (see, e.g., Patterson et al. 1996; Kato et al. 1996; Howell et al. 1996). The peak of the outburst was visually estimated to be 11.8 mag on April 5.83 UT (JD 2449813, see Howell et al. 1996).

We observed V=12.57 on April 6.87 UT (JD 2449814), a day after the maximum, and we followed the light curve decline during some photometric nights in April. Table 4 lists the $BVR_{\rm c}I\rm _c$ magnitudes while the $R\rm _c$ light curve is reported in Fig. 5.

  

Figure 5: $R\rm _c$ light curve of AL Com

The $BVR_{\rm c}I\rm _c$ light curves show a linear decay lasting for almost a week after the peak. We calculated the following values for the decay rates:

dM(B)/dt = 0.14$\pm$0.01 mag/day

dM(V)/dt = 0.14$\pm$0.01 mag/day

dM($R\rm _c$)/dt = 0.12$\pm$0.01 mag/day

dM($I\rm _c$)/dt = 0.12$\pm$0.01 mag/day.

In the V band, the rate is consistent with the values reported by Howell et al. (1996) and Patterson et al. (1996). It is worthy to note that the decay rate is slower at longer wavelengths. Nevertheless, during the superoutburst the colour indices remained quite stable around the mean values: (B-V)=-0.25, $(V-R\rm _c)=0.00$, $(V-I\rm _c)=0.03$.

At epoch 1995/05/02 UT (JD 2449839) AL Com was very faint. This point was taken during the temporary minimum reported by the above mentioned authors. Unfortunately we were not able to obtain a better sampled light curve of the variable due to bad weather conditions.

3.2 V544 Her

V544 Her was discovered and classified as a dwarf nova by Hoffmeister (1967). Also this star is very faint at quiescence ($m_{\rm pg} \simeq 20$), but during an outburst its photographic magnitude can increase up to 14.5 (Downes & Shara 1993). Howell et al. (1990) observed V544 Her at minimum and found the presence of a possible orbital modulation having a period of about 100 minutes.

We observed this dwarf nova during July 1995. The variable was found in a declining phase after a burst which, probably, reached the maximum a few days before 1995/06/27 UT (see Table 5). Although we were able to get only a few observations, we note that our observations on 1995/06/27 UT are the first $BVR_{\rm c}I\rm _c$ measurements of V544 Her during an outburst. At that time the colour indices were (B-V)=0.00, $(V-R\rm _c)=0.02$,$(V-I\rm _c)=0.23$.

3.3 V660 Her

The dwarf nova V660 Her is not well studied. As far as we know, only Shugarov (1975) observed this source during two outbursts. The first one was observed on August 4-5, 1973, while the second one occurred on August 12-13, 1974. In both the cases the estimated photographic magnitude of the variable at maximum was $m_{\rm pg} \simeq 16$.

We observed the dwarf nova V660 Her in July 1995 during the phase of decline from a large outburst. In Table 6 we report the $BVR_{\rm c}I\rm _c$ magnitudes while the light curve can be found in Fig. 6. The observed maximum was $B \simeq 14.3$ (JD 2449905), a value which is almost two magnitudes higher than that previously reported by Shugarov (1975). From our data it appears that the source probably reached its maximum a few days before our first positive detection (see Fig. 6), but the true maximum brightness is not likely to be higher by more than 0.5 mag.

  

Figure 6: $R\rm _c$ light curve of V660 Her

During the first 10 days the mean colour indices were (B-V)=-0.04, $(V-R\rm _c)=0.06$, $(V-I\rm _c)=0.07$, and the light curve showed a linear decay with the following rates:

dM(B)/dt = 0.14$\pm$0.03 mag/day

dM(V)/dt = 0.15$\pm$0.02 mag/day

dM($R\rm _c$)/dt = 0.13$\pm$0.01 mag/day

dM($I\rm _c$)/dt = 0.13$\pm$0.01 mag/day.

These values are quite similar to those recorded for AL Com and WZ Sge (Patterson et al. 1996).

At the end of the descending phase, the light curve showed a dip at the date 1995/07/18 UT (JD 2449917). This last part of the outburt, observed only in the $R\rm _c$ and $I\rm _c$ bands, seems to show a feature similar to the temporary minimum observed in AL Com (Howell et al. 1996) and WZ Sge (Patterson et al. 1996). The resemblance of the outburst light curve of V660 Her with AL Com and WZ Sge (especially the rates of decline which are very similar) and the large amplitude of the observed outburst suggest that V660 Her could belong to the recently identified sub-class of dwarf novae called "tremendous outburst amplitude dwarf novae'' (TOADs) (see Howell et al. 1995). However, further observations are required in order to know the true magnitude of V660 Her at minimum, now only estimated to be $m_{\rm pg} \simeq 19$ (Downes & Shara 1993), and to obtain a better sampling of the light curve during the outbursts.

3.4 V516 Cyg

The dwarf nova V516 Cyg was discovered and classified as a variable star by Hoffmeister (1949). Ahnert et al. (1949) classified V516 Cyg as an RW Aur variable. It was definitively classified as a dwarf nova by Meinunger (1966) and Erastova & Tsvetkov (1978). Spectroscopic observations of V516 Cyg have been published by Bruch & Schimpke (1992), Schimpke & Bruch (1992), Zwitter & Munari (1994). In the optical, the photographic magnitude is known to vary between $m_{\rm pg} \simeq 13.8$and $m_{\rm pg} \simeq 16.8$ (Downes & Shara 1993).

The $BVR_{\rm c}I\rm _c$ photometric observations were carried out during July 1995. The data are reported in the Table 7, while Fig. 7 shows the $R\rm _c$ light curve. As far as we know, this is the first time that V516 Cyg was observed during an outburst in the $BVR_{\rm c}I\rm _c$ bands. The variable remained bright for 3 days at a V magnitude of $\simeq$14, during which it had colour indices of (B-V)=0.1, $(V-R\rm _c)=0.2$, $(V-I\rm _c)=0.2$.

  

Figure 7: $R\rm _c$ light curve of V516 Cyg

3.5 DX And

DX And was discovered as a variable star by Romano (1958). It was classified as a dwarf nova of the U Gem type by Weber (1962, 1963). In the optical it has been studied by Romano & Perissinotto (1966), Bond (1978), Echevarria & Jones (1984), Bruch et al. (1987). Spectroscopic observations during quiescence were reported by Bruch (1989). The IUE spectra reported by Drew et al. (1991) show absorption features at 124 nm and 155 nm due to N V and C IV respectively. Drew et al. (1993) determined a binary period of 0.44167 day and a mass ratio of 0.96, through spectroscopic observations when DX And was at minimum. Photometric observations in the Johnson B-band, Kron-Cousins $R\rm _c$-band, and Gunn z-band revealed ellipsoidal variation of 0.08 mag only in Gunn z (Drew et al. 1991). Analysing the luminous variations in the $R\rm _c$ and $I\rm _c$ bands, Hilditch (1995) suggested that the inclination of the orbital plane of DX And is in the range between $45\hbox{$^\circ$}$ and $53\hbox{$^\circ$}$, and that the secondary component is a K0-1 V star.

Although DX And is one of the better studied cataclysmic variables in our sample, up to date only few photometric data obtained during the phase of the outburst have been published. The interval between two consecutive outbursts varies between 8-12 months (see Drew et al. 1993).

We monitored DX And intermittently during the second half of 1994. It was always at quiescence, until September 28, 1994 UT (JD 2449624), when the dwarf nova was found to be in outburst. Our data are reported in Table 8. The $I\rm _c$ light curve is shown in Fig. 8.

  

Figure 8: $I\rm _c$ light curve of DX And

We note that the outburst amplitude is larger at higher frequencies: that is $\Delta B\simeq 4$, $\Delta V\simeq 3$, $\Delta R\rm _c\simeq 2.5$, $\Delta I\rm _c\simeq 2$.

Soon after the maximum, DX And showed a non-linear decay, starting with a slow rate which progressively became faster. In the last part of the decline (from JD 2449631 to 2449634) the decay appear to be approximately linear, with the following rates:

dM(B)/dt = 0.37$\pm$0.01 mag/day

dM(V)/dt = 0.34$\pm$0.01 mag/day

dM($R\rm _c$)/dt = 0.30$\pm$0.01 mag/day

dM($I\rm _c$)/dt = 0.28$\pm$0.01 mag/day.

To study the rapid variability of DX And, we performed time series observations during two nights: the first one during the decline (JD 2449632) and the second one during the quiescent phase (JD 2449639). We used only the $I\rm _c$ filter and the data are displayed in Fig. 9. During the first run we saw a regular descending trend with a rate of 0.23$\pm$0.03 mag/day. In the second run we found a rapid variation of about 0.1 mag similar to the ellipsoidal variability observed by Hilditch (1995) during the quiescence, while this effect is not evident during the decline probably because of the presence of the bright accretion disk.

  

Figure 9: Intranight light curves of DX And during the decline (top) and at the minimum (bottom)