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\IBVShead{4335}{2 May 1996}

\title{Observations of Low-amplitude Late-Type Variables}


\begintext

The low-amplitude red-giant variables UX~Dra, $\delta^{2}$ Lyr, VY~UMa and
RR~UMi
have been observed almost continuously since mid-1994 as
part of a programme to investigate known and suspected red variables.
UX~Dra and VY~UMa are very similar stars; they are both luminous
carbon stars with Tc (Little \etal\ 1987) while
RR~UMi and $\delta^{2}$ Lyr are less evolved AGB stars
(Jorissen \etal\ 1993, Eggen 1993).

The
observations were made using an SSP3 photometer and nominal V filter on a
20-cm Newtonian reflector.
Each observation consisted of 2 or 3 sets of 3$\times$10 second
integrations.
Differential extinction corrections were applied but these are small.
Details of the comparison stars are given in Table 1.
For further information please contact the authors.

UX Dra (HD\,183556, SAO\,9404, BD\,+76\degr 734) has a spectral type of C5\,II and according to the GCVS (Kholopov \etal\ 1985)
is an SRa with a period of 168 days.
From times of minimum, Vetesnik (1983) found a constant increase of
the period but more extensive photographic data suggested that this was part
of a long term cycling of the period between 155 and 185 days, over about
5000 days.
In the light curve of UX Dra (Figure 1)
the largest variations have a period of 168 days
superimposed on a large gradient.
However, the earliest observations show a variation of only $\sim$0.1 mag on a
time scale of 50--100 days. As the published 168 day period appears in these data, it seems unlikely~that
\vskip 2mm
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\centerline{\epsffile{4335-f1.ps}}
\centerline{Figure 1. Light curve of UX Dra}
\vfill \eject

\centerline{2}
\vskip 1mm
\epsfysize=8cm
\centerline{\epsffile{4335-f2.ps}}
\centerline{Figure 2. Light curve of $\delta^{2}$ Lyr}
\vskip 4mm
\noindent  the period is variable. The time of lower activity may indicate that the
behaviour suggested by Vetesnik is due to phase
changes in this relatively stable period.

$\delta^{2}$ Lyr (HR\,7139, HD\,175588, SAO\,67559) is a fourth magnitude star
of spectral type M4\,II.
Photometry during the 1980's led Bakos \& Tremko (1990) to conclude that
it is a semi-regular variable with a
characteristic time scale in the range 50--100 days.
Superimposed on this are longer term variations in the mean
magnitude and shorter term excursions of similar amplitude.
On the basis of the variation in the annual light curves Bakos \& Tremko
were unable to suggest a single period but concluded that
the period was variable.
The dominant feature of the new observations (Figure 2) is a strong
periodicity at
70--80 
\epsfysize=8cm
\centerline{\epsffile{4335-f3.ps}}
\begin{center}Figure 3.
DFT power spectrum of the new photometry of ${\delta}^{2}$ Lyr combined with that of Bakos \& Tremko
\end{center}
\vfill \eject

\centerline{3}
\vskip 1mm
\centerline{Table 1. Comparison star information}
\begin{center}
\begin{tabular}{lllrlrr}
\hline
Variable & ~~~~~~Comparison & ~$V$ & $B-V$ & ~Sp & $\Delta V$~ & $\sigma$~~ \\
\hline
UX Dra & HR\,7199 = HD\,176795 & 6.22: &      & A1\,V & $-$0.233 & 0.016\\
       & HR\,7247 = HD\,178089 & 6.47 & 0.45 & F2\,V  \\
\hline
$\delta^{2}$ Lyr
       & HR\,7131 = HD\,175426 & 5.58 & $-0.15$ & B2.5\,V & 0.302 & 0.022 \\
       & HR\,7174 = HD\,176318 & 5.89 & $-0.17$ & B7\,IV &  \\
\hline
VY UMa & HR\,4176 = HD\,92523  & 5.75 & 1.30 & K3\,III & $-$0.740 & 0.017 \\
       & HR\,4181 = HD\,92523  & 5.00 & 1.38 & K3\,III & \\
\hline
RR UMi & HR\,5691 = HD\,136064 & 5.13 & 0.53 & F9\,IV & 1.065 & 0.018 \\
       & HR\,5629 = HD\,133994 & 6.13: &      & A2\,Vs & \\
\hline
\end{tabular}
\end{center}
\vskip 4mm
\noindent days, but many of the characteristics seen previously are also visible.
The periodogram of the combined photometry (Figure 3)
is dominated by a long term variation, which is not necessarily periodic
and a period of 79 days ($f$ = 0.01267 cycles\,day$^{-1}$),
together with their one year aliases.
$\delta^{2}$ Lyr has a complex light curve with many time scales, but
the 79 day period is sufficiently stable
to appear in observations covering the past 10 years and is
the dominant short-term feature of the data.

VY UMa (HR\,4195, HD\,92839, SAO\,15274, BD\,+68\degr 617) is of spectral type
C5\,II and is given by the GCVS as an Lb variable. Analysis of
visual data covering 1990--1993 shows a period of 120 days
(Ofek, Shemmer \& Gabzo 1995).
The AAVSO photoelectric observations, which are  admittedly rather 
sparse, suggest a
variation on a time scale of $\sim$200 days (Percy \etal\ 1996) but these
cover the years 1986--1992 when the behaviour may have been different.
The periodogram of the new data in Figure 4 shows a clear period at 118
days, and this is a remarkably good fit to most of the data.
The amplitude is clearly variable but, for the most part,
the period is relatively stable,
and even after there is some disruption the
phasing is retained.
As there is no overlap between these data and the visual observations it
seems likely that the 118 day period dominated for most of the past six
years.
However, the absence of this period in the
AAVSO data suggests that it was probably not dominant before that.

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\centerline{\epsffile{4335-f4.ps}}
\centerline{Figure 4. Light curve of VY UMa with the 118 day period superimposed}
\vfill\eject

\centerline{4}
\vskip 1mm
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\centerline{\epsffile{4335-f5.ps}}
\centerline{Figure 5. Light curve of RR UMi}
\vskip 4mm
RR UMi (HR\,5589, HD\,132813, SAO\,16558, BD\,+66\degr 878) has a spectral type of M4.5\,III and is one of the few
semi-regular variables in a binary with a known orbit (Batten \& Fletcher
1986). The period is 749 days but there is no indication of the secondary.
RR UMi is listed in the GCVS
as an SRb with a period of 43.3 days but
variations on a range of
time scales from 30--50 days have been found by Percy \etal\
(1994).
The periodogram of the light curve in Figure 5
shows no dominant period, although periods of
$\sim$61 and $\sim$34 days are present.
A suggestion of the 61 day period can be followed through the first half of
the data but then becomes quickly lost.
During the second half of the data the time scale of the variation halves
which is probably responsible for the 34 day period in the periodogram.
%and the average magnitude brightens slightly.
RR UMi shows variation on the 30--60 day time scale with no
dominant period in the present data.
There is no indication of a variation with a period of 43 days.


\vskip .8cm
\beginauthors
C. LLOYD
Rutherford Appleton Laboratory
Chilton, Didcot
OXON OX11 0QX, UK
{\tt cl@ast.star.rl.ac.uk}                        %email
\authorsrightcol
K. W. WEST
5, Edward Street
Ryde
Isle of Wight PO33 2SH, UK
{\tt kwest@aladdin.co.uk}                        %email
\endauthors

\references

Bakos, G.A. \& Tremko, J., 1991, {\it Contr. Astron. Obs. Skalnate Pleso}, {\bf  21}, 99
% Original: Bakos, G.A. \& Tremko, J., 1990, {\it Contr. Astron. Obs. Skalnate Pleso}, {\bf  21}, 99
% Corrected: Mon Jul 28 16:46:27 MET DST 2003

Batten, A.H. \& Fletcher, J.M., 1986, {\it PASP}, {\bf 98}, 647

Eggen, O.J., 1993, {\it AJ}, {\bf 104}, 275

Jorissen, A. \etal, 1993, {\it A\&A}, {\bf 271}, 463

Kholopov, P.N. \etal, 1985 {\it General Catalogue of Variable Stars},
4th edn., Moscow

Little, S.J., Little-Marenin, I.R. \& Bauer, W.H., 1987, {\it AJ}, {\bf 94}, 981

Ofek, E.O., Shemmer, O. \& Gabzo, O., 1995, {\it J. BAA}, {\bf 105}, 33

Percy, J.R. \etal, 1994, {\it PASP}, {\bf 106}, 611

Percy, J.R., Desjardins, A., Yu L. \& Landis, H.J., 1996, {\it PASP}, {\bf 108}, 139

Vetesnik, M., 1983, {\it IBVS}, No.\,2329

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