In active galactic nuclei (AGNs) at least two types of variability may be present, i.e. the short-term and the long-term variations. The short-term variation is probably non-periodic although a short-term period has been indeed observed in our monitoring programme of BL Lac objects (Xie et al. 1988, 1990), the long-term variation has in some cases claimed to be periodic. For example, the 11.4 years period which Webb et al. (1988) found in the light curve of 3c 345 successfully predicted the 1991 outburst of the object (Kidger & Takalo 1990), the 12 years period found in BL Lac object OJ 287 (Sillanpaa et al. 1988; Kidger et al. 1992) also successfully predicted the optical outburst in the fall of 1994 of the object (Sillanpaa et al. 1996). To establish the reality of such long periods two prerequisites must be satisfied. First, the data sample must be of very long duration: at least six times the length of the periodicity being claimed would seem to be a valid criterion for demonstrating that any possible period is not simply a random event and probably has some physical significance. Secondly, the possible period should be of large amplitude and thus directly visible in the light curve; where the amplitude is low, many more than six cycles of the period will be necessary to demonstrate the reality. Some AGNs seem to satisfy these two prerequisites and have shown period variations (ON 231 Liu et al. 1995; OJ 287, Sillanpaa et al. 1988a; Kidger et al. 1992; Sillanpaa et al. 1996).
PKS 0735+178 has been classified as a BL Lac object by Carswell et al. (1974).
Its measured redshift
is based on a simple absorption doublet of Mg II (Burbidge & Hewitt 1987).
Observations show it
has a flat radio spectrum and radio bursts (Ulvestad & Johnston 1984;
Antonucci & Ulvestad 1985;
Baath & Zhang 1990; Zhang et al. 1991;
Terasranta et al. 1991, 1992;
Stevens et al. 1994), violent
optical variations (Pollock et al. 1979;
Worrall et al. 1981; Miller
1983; Xie et al. 1992), high
polarization and superluminal motion (Aller et al. 1985;
Wardle &
Roberts 1988; Porcas 1987; Baath
& Zhang 1991; Briadle et al. 1986;
Impey et al. 1987):
%,
%,
%.
Bregman et al. (1984)
also noticed that its infrared polarization
is associated with flux. Webb et al. (1988) and Smith et al. (1987)
constructed its historical
optical light curve, searched for its periodicity using the Fourier
analysis and found periods of
1.2 and 4.89 years. From the paper of Kidger et al. (1992), we have
that the Fourier analysis is
not as good as the Jurkevich's method to find periodicities. So, we have
used the powerful method
of Jurkevich (Jurkevich 1971; Kidger et al. 1992) to determine the
periodicity of PKS 0735+178.
In this paper, we mainly show the optical variability with some new observations presented and discuss its periods by means of the method (Jurkevich 1971). The paper has been arranged as follows: In Sect. 2 we give the Jurkevich method, in Sect. 3 the observation data including our new data, and in Sect. 4 discussion and results.