1 Introduction

 

Table 1: The journal of observations. (R - heliocentric distance; $\Delta$ - geocentric distance; E - elongation; $\lambda$ - ecliptic longitude; $\beta$ - ecliptic latitude; $\alpha$ - solar phase angle; aspect data are referred to 2000.0; $\mu _R$ - mag/arcsec² in R-band).

Date	RA	Dec	R(AU)	$\Delta$(AU)	E	$\lambda (^\circ)$	$\beta (^\circ)$	$\alpha (^\circ)$	see( $^{\prime\prime}$)	sky($\mu _R$)	exp(s)
C/1999 F2
2000 June 30	17 05	+30 45	5.512	4.728	137.6	205.3	63.7	7.3	2.1	20.2	240
C/1999 J2
2000 July 06	15 20	+32 29	7.135	6.836	104.2	326.2	48.1	8.0	1.5	19.8	240
2000 July 09	15 19	+32 03	7.136	6.868	101.9	326.1	47.7	8.0	2.1	19.5	240
C/1999 N4
2000 July 04	16 23	+05 35	5.514	4.771	133.9	297.2	26.7	8.0	1.6	20.7	240
2000 July 06	16 21	+05 32	5.515	4.794	132.0	297.2	26.7	8.0	1.4	19.8	240
C/1999 T2
2000 July 02	21 29	+60 25	3.351	3.318	90.5	249.6	53.3	17.7	1.9	20.2	90
C/2000 H1
2000 June 30	15 27	+48 18	3.890	3.643	96.4	347.4	66.7	15.1	2.1	20.2	240
C/2000 K1
2000 July 04	16 04	+13 05	6.429	5.798	125.4	304.4	33.2	7.5	1.6	20.7	180

The overwhelming majority of cometary studies are based on ground-based or space observations of bright comets around or near to their perihelion. Direct imaging reveals the inner structure of the coma, which usually hides the nucleus itself. There has been a small number of papers dealing with distant ($R\geq5$ AU) comets (e.g. O'Ceallaigh et al. 1995; Lowry et al. 1999), and consequently, there is a serious lack of information on the behaviour of these objects. On the other hand, time-series observations may give constraints on the period of rotation and related effects (Jewitt 1992).

Optical photometry of sunlight scattered from the nucleus was first attempted by Fay & Wisniewsky (1978) on comet 6P/d'Arrest. The approach is identical to that used in the asteroid studies. Further CCD photometry using very small aperture radii has been used to study the rotation of some comets by Jewitt & Meech (1985), Jewitt (1990), Meech et al. (1993, 1997), Licandro et al. (2000a, 2000b). Licandro et al. (2000a) discussed in detail rotation and shape models based on the lightcurves. Additional reduction has to be done with respect to the seeing effects as the variation in seeing may cause a virtual light variation of the inner coma with an amplitude of some tenths of a magnitude.

The nucleus is usually well embedded in the coma and two basic approaches exist to determine the nuclear diameter. The most efficient one is the use of high spatial resolution observations of closely passing comets with the HST (e.g. Lamy & Tóth 1995; Lamy et al. 1998, 1999). The other commonly used method is observing distant nuclei in quiescence, when the coma is effectively absent (Luu & Jewitt 1992; O'Ceallaigh et al. 1995; Boehnhardt et al. 1999). Very recently, the nuclear thermal radiation of some comets was studied in the infrared with help of ISOCAM observations, resulting in independent diameter estimates (Jorda et al. 2000).

The new observing strategies by automatic telescopes (e.g. project LINEAR) and dedicated large instruments (Hainaut & Meech 1997) led to regular discoveries of relatively bright, large perihelion-distance comets, often more months or a year before the perihelion passage. Thus, one can study cometary activity in such regions that were unavailable even a decade ago. The main aim of our work is to contribute to this topic with new CCD observations of three comets located between 5.5-7.2 AU - C/1999 J2 (Skiff), C/1999 N4 (LINEAR) and C/2000 K1 (LINEAR). Beside the absolute $VR_{\rm C}$ photometry we took time-series observations in order to detect rotational effects. The paper is organised as follows. The observations are described in Sect. 2, while Sect. 3 deals with the analysis and detailed observational results. A brief discussion is given in Sect. 4.