2 Observations and results

2.1 Photometry

The photometric observations were made in 1984-1996 at four different sites within the frame of a joint observational program. We used the following telescopes: 0.6 m Swedish telescope at La Palma (LP), Canary Islands, 61 cm Bochum, 50-cm and 1 m ESO telescopes at ESO, La Silla, Chile and the 70-cm telescope of Astronomical Observatory of Kharkiv State University (KhAO). The observations at KhAO and with the Bochum telescope were CCD observations, otherwise conventional photoelectric photometry was used. All the photometric reductions were made with standard methods. The data are corrected for light-time. Table 1 contains the aspect data of the observed asteroids and references to the used telescopes. The obtained rotation periods and amplitudes are given in Table 2. The errors in the rotation periods are indicated by the number of decimals given. The amplitude errors are normally of a few hundreds of a magnitude. Figures 1-11 present the composite lightcurves of individual asteroids. Below we discuss each asteroid in more detail.

  

Table 1: Aspect data

  

Table 2: Rotational periods and lightcurve amplitudes

97 Klotho:

This asteroid has previously been observed during six apparitions. Harris & Young (1983) determined a rotation period of  35 hours and pointed out that this was the longest period among the known M-type objects. Lagerkvist et al. (1988) determined a rotation period of 35.58 hours. Data obtained by Dotto et al. (1992) agree with this period but they do not cover a whole rotation cycle. The extensive observational run undertaken by Lagerkvist et al. (1995) confirmed the slow rotation (35.0 hours) of 97 Klotho. Our observations during three apparitions also give a long rotation period. The observations in 1995, made during March-May, gave us a possibility to define a more precise value of the rotation period. A period of 35.15 hours is the best solution from our observations (Fig. 1). All previously obtained data of 97 Klotho agree well with this rotation period.

  

Figure 1: Composite lightcurve of 97 Klotho

217 Eudora:

This asteroid was classified to be of type M by Belskaya et al. (1991) based on its colour indices and the value of minimum polarization (0.82$\%$), which are typical for M-type objects. The IRAS albedo (0.05), however, is more typical for P-type asteroids. Since the agreement between polarimetric and radiometric albedos is poor we included the asteroid in the list of possible M-type objects. From our observations during three nights we estimate a rotation period of 12.54 hours (Fig. 2) as the most probable one assuming a standard lightcurve with two pairs of extrema. Unfortunately, the obtained data do not give a possibility to determine the rotation period uniquely.

  

Figure 2: Composite lightcurve of 217 Eudora

322 Phaeo:

Harris & Young (1983) observed 322 Phaeo but only during a short interval of time. They found only minor changes of the magnitude and concluded that the rotation period was long. Our observations during four nights gave an amplitude of about 0.2 mag and an unambiguous rotation period of 17.56 hours (Fig. 3).

  

Figure 3: Composite lightcurve of 322 Phaeo

337 Devosa:

This asteroid has previously been observed during eight apparitions (see Lagerkvist et al. 1996, for detailed references). Lightcurves of 337 Devosa show an asymmetrical shape with three pairs of extrema. We observed 337 Devosa in September 1995 during more than a complete rotational cycle in the B and V bands. The lightcurve in the V band is shown in Fig. 4. The scatter around rotational phase 0.6 was caused by varying photometric conditions during the night.

  

Figure 4: Composite lightcurve of 337 Devosa

558 Carmen:

The asteroid 558 Carmen was previously observed during three apparitions by Harris & Young (1979, 1989) and Harris et al. (1992) who estimated the period to 10 hours. Our observations during two nights agree with this period (Fig. 5).

  

Figure 5: Composite lightcurve of 558 Carmen

572 Rebekka:

Our observations during three nights give a rotation period of 5.65 hours. The composite lightcurve is quite normal with two pairs of extrema and the lightcurve amplitude is 0.3 mag (Fig. 6).

  

Figure 6: Composite lightcurve of 572 Rebekka

757 Portlandia:

From our observations during five nights we determined an unambiguous rotation period of 6.58 hours. The lightcurve is irregular and its shape changed noticebly as the phase angle decreased (Figs. 7 and 8).

  

Figure 7: Composite lightcurve of 757 Portlandia. The phase angle is high, 21$^\circ$-23$^\circ$

  

Figure 8: Composite lightcurve of 757 Portlandia obtained at much lower phase angles than in Fig. 7 (6$^\circ$-7$^\circ$)

857 Glasenappia:

Our observations during three nights give a rotation period of 8.23 hours and a lightcurve amplitude of 0.35 mag. The composite lightcurve shown in Figure 9 is regular with two pairs of extrema.

  

Figure 9: Composite lightcurve of 857 Glasenappia

  

Figure 10: Composite lightcurve of 872 Holda

  

Figure 11: Composite lightcurve of 872 Holda

872 Holda:

We observed 872 Holda during two apparitions, each time during two nights. The obtained lightcurves are rather regular with amplitudes of 0.2-0.4 mag depending on the aspect angle. Two possible values of the rotation period were determined: 6.78 or 7.20 hours. Composite lightcurves based on a period of 7.20 hours are shown in Figs. 10 and 11. More observations are needed to define an unambiguous value of the rotation period.

2.2 Colour indices of selected asteroids

In order to increase the sample of known M-type asteroids we observed 17 previously unclassified asteroids for which good-quality IRAS albedos were known. For our purpose we chose asteroids with moderate albedo which are well-separated into the taxonomic types S, M, A and V according to their B-V colour index (Zellner & Bowell 1979). The observations were carried out in the standard B and V bands using the 1-m telescope at ESO, Chile in 1993 and the 61cm Bochum telescope at ESO during 1994. During each night a set of standard stars were carefully observed. Table 3 gives the asteroid number and name, the measured magnitude in the V band, the B-V colour index, albedo and diameter according to IRAS data (Tedesco et al. 1992) In the last column we give our classification of these asteroids. Seven asteroids have B-V colours within the range of the M-type population, eight asteroids with the S-type and one asteroid have a B-V colour at the border of the M and S type populations. The asteroid 1562 Gondolatch is characterized by an extremely large B-V =1.04 mag which indicates that it is of type A. At size ranges larger than 50 km the ratio between S and M-type asteroids is close to three (Zellner & Bowell 1979). However, in our sample, which is mainly composed of objects smaller than 50 km, this ratio is two. We do not want to draw too firm conclusions from the statistics with small numbers but there is a possibility that for smaller asteroids the proportion of M type asteroids is higher than for larger objects.

  

Table 3: Results of B-V measurements