The observational material

For the present study only archival light curves of the four systems HT Cas, V2051 Oph, IP Peg and UX UMa observed at the SAAO, South Africa, at the MacDonald Observatory, USA, and at the Wise Observatory, Israel, were used. Most of the data of HT Cas and V2051 Oph were already studied with respect to other properties than the flickering. With the exception of two so far unpublished light curves the data of HT Cas were published by [Patterson (1981)] and [Zhang et al. (1986)]. Of V2051 Oph, 6 light curves are unpublished; the others can be found in [Warner & Cropper (1983)] and [Warner & O'Donoghue (1987)]. The quoted references also contain details about the observations. In contrast, none of the light curves of IP Peg and UX UMa used here was published to my knowledge.

The large majority of the data refers to white light, but a few light curves were observed with the four channel Stiening photometer (Horne & Stiening 1985). In these cases only the B band was employed. The brightness of the stars is given as counts per integration time, corrected for extinction and sky background. The latter features imply that only an approximate correction of the scatter curves for Poisson noise is possible. However, this does not affect the results seriously (see Bruch 1996 for a discussion of this point). For the three dwarf novae in the present sample (HT Cas, V2051 Oph and IP Peg) only light curves obtained during quiescence are used.

The amount of available data differs very much for the four stars investigated here. Whereas data of 66 orbital cycles were used in the case of V2051 Oph, the corresponding numbers for HT Cas, UX UMa and IP Peg are 22, 9 and 3. Since the dependence of the scatter in the light curves is to be studied here as a function of orbital phase all light curves were phase folded (after splitting those light curves containing more than one cycle into separate curves). Table 1 contains an overview: The individual columns list the cycle number, the heliocentric Julian Date of the corresponding eclipse, the civil date, and the time resolution of the light curves in seconds. The phase coverage of the individual cycles is not always complete. Whereas for HT Cas and V2051 Oph in most cycles the complete phase range -0.5 < phi < 0.5 around the eclipses is covered, in the longer period system UX UMa in general only a restricted phase range centered on the eclipse was observed.

 

 

Table 1: Eclipse numbers, Julian dates, civil dates and time resolution for the light curves used in this study

E	HJD	civil date	Delta t	E	HJD	civil date	Delta t	E	HJD	civil date	Delta t
	2440000+		(sec)		2440000+		(sec)		2440000+		(sec)
HT Cas:				V2051 Oph (cont.):				V2051 Oph (cont.):
0	3727.93721	1978/8/7	3	4873	5091.53210	1982/5/2	5	17446	5876.43759	1984/6/24	3
448	3760.93116	1978/9/9	3	5145	5108.51248	1982/5/19	5	17460	5877.31158	1984/6/25	3
461	3761.88857	1978/9/10	3	5207	5112.38301	1982/5/22	5	17461	5877.37400	1984/6/25	3
462	3761.96222	1978/9/10	3	5223	5113.38185	1982/5/23	5	17462	5877.43643	1984/6/25	3
733	3781.92061	1978/9/30	3	5289	5117.50209	1982/5/28	10	17463	5877.49886	1984/6/25	3
1599	3845.69909	1978/12/3	5	5336	5120.43620	1982/5/30	10	17475	5878.24799	1984/6/26	5
1600	3845.77274	1978/12/3	5	5351	5121.37262	1982/5/31	10	17476	5878.31042	1984/6/26	5
1897	3867.64896	1978/12/25	8	5352	5121.43505	1982/5/31	1	17477	5878.37285	1984/6/26	5
1898	3867.71963	1978/12/25	8	15591	5760.63391	1984/3/1	5	17478	5878.43528	1984/6/26	5
1993	3874.71609	1979/1/1	3	16632	5825.62131	1984/5/5	5	17492	5879.30927	1984/6/27	5
5105	4103.90619	1979/8/18	8	16676	5828.36813	1984/5/7	5	17572	5884.30350	1984/7/2	5
6853	4232.64150	1979/12/25	4	16677	5828.43056	1984/5/7	5	17573	5884.36592	1984/7/2	5
6854	4232.71515	1979/12/25	4	16678	5828.49299	1984/5/7	5	18293	5929.31398	1984/8/16	5
20773	5257.81058	1982/10/15	2	16679	5828.55542	1984/5/8	5	18294	5929.37641	1984/8/16	5
20786	5258.76799	1982/10/16	1	16680	5828.61785	1984/5/8	5	24044	6288.33661	1985/8/10	1
20787	5258.84164	1982/10/16	1	16694	5829.49184	1984/5/8	10	24045	6288.39903	1985/8/10	1
20813	5260.75647	1982/10/18	1	16695	5829.55426	1984/5/9	10	24156	6295.32853	1985/8/17	5
20828	5261.86118	1982/10/19	1	16696	5829.61669	1984/5/9	10
21573	5316.72834	1982/12/13	1	17031	5850.53002	1984/5/30	5
21598	5318.56952	1982/12/15	2	17032	5850.59245	1984/5/30	5	IP Peg:
21599	5318.64317	1982/12/15	2	17045	5851.40401	1984/5/30	5	?	?	1990/10/26	2
21600	5318.71682	1982/12/15	2	17046	5851.46644	1984/5/30	5	?	?	1990/10/26	2
				17047	5851.52887	1984/5/31	5	21101	8953.73058	1992/11/27	1
				17061	5852.40286	1984/5/31	5
V2051 Oph:				17062	5852.46529	1984/5/31	5
								UX UMa:
-23044	3348.73354	1977/7/24	3	17063	5852.52772	1984/6/1	5
-17933	3667.80233	1978/6/8	3	17064	5852.59014	1984/6/1	5	-6897	2548.43692	1975/5/15	5
-16910	3731.66603	1978/8/11	4	17078	5853.46413	1984/6/1	5	-6892	2549.42027	1975/5/16	5
-11807	4050.23540	1979/6/26	4	17079	5853.52656	1984/6/2	5	-6887	2550.40363	1975/5/17	4
0	4787.32114	1981/7/1	5	17080	5853.58899	1984/6/2	5	-6882	2551.38698	1975/5/18	5
								-3016	3311.71815	1977/6/17	5
1	4787.38357	1981/7/1	5	17396	5873.31619	1984/6/21	3
17	4788.38241	1981/7/2	5	17429	5875.37631	1984/6/23	3	477	3998.69092	1979/5/5	5
18	4788.44484	1981/7/2	5	17430	5875.43874	1984/6/23	3	482	3999.67428	1979/5/6	5
64	4791.31652	1981/7/5	5	17431	5875.50117	1984/6/24	3	487	4000.65763	1979/5/7	5
4265	5053.57596	1982/3/25	5	17443	5876.25030	1984/6/24	3	24787	8779.76969	1992/6/6	1
4329	5057.57135	1982/3/29	5	17444	5876.31273	1984/6/24	3
4872	5091.46967	1982/5/1	5	17445	5876.37516	1984/6/24	3

The cycle numbers and the corresponding eclipse numbers quoted in Table 1 are based on the ephemeris of Horne et al. (1991) in the case of HT Cas, [Echevarría & Alvarez (1993)] for V2051 Oph and [Baptista et al. (1995)] for UX UMa. The orbital period of IP Peg is not stable as was first pointed out by [Wood et al. (1989b)]. [Wolf et al. (1993)] later found the period variations to be cyclic. Here, the ephemeris of [Wolf et al. (1993)] are adopted. However, in two cases none of the published ephemeris predicts the epochs of the minima observed in the present light curves even approximately. There may be a timing error. Therefore, no cycle numbers and eclipse epochs are given for these cases in Table 1.

Not in all cases the eclipse epochs predicted from the ephemeris matched well the observed eclipses. Instead of trying to trace the reasons for the inconsistencies (difficult in view of the limited information available about the observational details of the archival data) a pragmatic approach was adopted: In such cases, the epoch of the eclipse centre was visually measured in the light curve (sufficient to ensure an accuracy which is higher than the phase resolution of the final scatter curves), and the phase-folding was done using that epoch as zero-point.