PKS 0735+178 has been observed for 90 years back to JD 2417233. Optical
observations indicate that
not only does it have large variation with long term scale (B=13.90-17.22)
(Pollock et al. 1979)
but also it has a short-time scale variation:
weak (McGimsey et al. 1975);
day
(Miller 1983) and
0.68/24 min (Xie et al. 1992).
Our group have also monitored the object
since 1982 (Xie et al. 1987, 1988, 1992, 1994). During December
1994, January 1995, and January and
February 1996, it was observed with the 102 cm RCC telescope at Yunnan
Observatory and the 156 cm
RCC telescope at Shanghai Observatory, which were equipped with direct
CCD cameras. The daily
averaged magnitudes are given in Table 1 (click here).
All magnitudes observed from
it will be published in a
separate paper altogether with other BL Lac objects observed during
the period (Xie et al. 1997).
From our recent observation, PKS 0735+178 variated in a range of
16.89 (JD 2449691.65)-15.77
(JD 2450107.5) magnitudes in B band. In this paper the historic
optical data in B band (434 points)
are from the literatures (Wing 1973; Carswell et al. 1974;
Pollock 1975;
O'Dell et al. 1978; Zekl et
al. 1981; Moles et al. 1985; Smith et al. 1987b;
Webb et al. 1988;
Sillanpaa et al. 1988b; Tanzi
1989; Takalo 1991, 1992; Valtaoja et al. 1993) with the 
in the paper of Zekl et al. (1981) changed
into B by 
(Kidger 1989; Lu 1972). Also those
observed by our group (Xie et al. 1987,
1988, 1992, 1994) and those in the present paper (Table 1 (click here)) are shown in
Fig. 1 (click here). Observation indicate
that there was a large variation of about 4.6 magnitudes during
JD 2400 000+43200 and 44200. From
the UBVRI data in the literatures we have 
(67
pairs data), 
(54),
(55), 
(55),
the errors are the 1 sigma dispersion of the data.
| UT Date | B | V |
| day/mon/yr | Mag. | Mag. |
| 04/12/94 | 16.89  0.10 | 15.92 0.08 |
| 05/12/94 | 16.90 0.10 | 16.02 0.08 |
| 06/12/94 | 16.85 0.10 | 15.97 0.08 |
| 23/01/95 | 16.03 0.14 | 15.30 0.06 |
| 25/01/95 | 16.46 0.08 | --------- |
| 26/01/95 | 16.01 0.05 | 15.35 0.09 |
| 27/01/95 | 16.02 0.08 | 15.50 0.06 |
| 28/01/95 | 15.89 0.08 | 15.46 0.04 |
| 29/01/95 | 15.96 0.09 | 15.48 0.14 |
| 24/01/96 | 15.77 0.08 | --------- |
| 15/02/96 | --------- | 15.60 0.04 |
Figure 1: The light curve in B band of PKS 0735+178 for about 90 years
(434 points)
To counteract the heavy weighting of the recent data, we have calculated 10-day means of the light curve. This interval is short enough compared to the long-term periods (years) and thus unlikely to distort the long-term variations too much. This light curve consists of 218 points and is shown in Fig. 2 (click here). When the method in Sect. 2 is used to the averaged data (m =10), two strong periods: 14.2 years (Vm2 = 0.637, f = 0.570); 28.69 years (Vm2 = 0.599, f = 0.669) have been found(see Fig. 3 (click here) for detail). The former is about half the latter, and they are in fact the same with the latter being just a harmonic of the former.
Figure 2: The mean light curve in the form of 10-day means (218 points)
Figure 3: The normalized Jurkevich test results for the period search for BL Lac
object PKS 0735+178 using the data shown in Fig. 2 with m=10