10 Hygiea is a C-type asteroid (Tholen 1989) with a diameter of 429 km (Tedesco 1989). 10 Hygiea has been observed in eight oppositions between 1953 and 1991 (Groeneveld & Kuiper 1954; Vesely & Taylor 1985; Lagerkvist et al. 1987b, 1989, 1992c; Michalowski et al. 1991), and now in 1996. 10 Hygiea has been also observed by ISO satellite (Dotto et al. 1998, 1999) and imaged with the Hubble Space Telescope (Storrs et al. 1998, 1999). No companions to 10 Hygiea have been detected down to a limit of seven magnitudes.

From our data we find a
synodic period for 10 Hygiea of 27.63
0.02 hours.
This synodic period is consistent with the sidereal period derived by
Michalowski et al. (1991)
from all the observations of 10 Hygiea between 1953 and 1989, and also
with the
observations in 1991 (Erikson & Magnusson 1993).
Figure 1 shows the composite lightcurves of 10 Hygiea derived using this
synodic period.
Although there is a gap in the coverage of the rotational phase of the
lightcurve of this asteroid at the time of one of the minima,
the composite lightcurves obtained for this asteroid show
two maxima and two minima per rotational cycle in all the *uvby* filters.
The two maxima are of different magnitude by
.
The maximum amplitude is of
in all the *uvby* filters.

The *b*-*y* colour index does not show any variation during the rotational phase
of this asteroid (see
Fig. 1). The *v*-*b* and *u*-*b* colour indices seem to show a
variation during the rotational phase of this asteroid.
These *v*-*b* and *u*-*b* colour curves seem to be
anticorrelated with the *uvby* lightcurves,
although
the lack of data that overlap during fractional parts of the rotational
phase of this asteroid does not let us to conclude
that these detected variations in the *v*-*b* and and *u*-*b* colour curves
are anticorrelated with the *uvby* lightcurves.
Recently, rotational spectral variability of 10 Hygiea,
in the 0.7 m region, has been detected
by Howell et al. (1999).

The mean values of and found for this asteroid, agree with the values, and , reported by Bowell et al. (1979).

The results obtained here for the sidereal period, pole and shape of
10 Hygiea are
being
,
(or
,
),
*a*/*b*= 1.31, *b*/*c*= 1.20 and
,
having
a retrograde sense of rotation,
in agreement with the later determinations (Michalowski et al.
1991; Michalowski 1993; Erikson & Magnusson 1993).

The observed amplitudes
together with the theoretical amplitudes, at zero-phase angle,
obtained with the solution values of *a*/*b* and *b*/*c* versus the aspect angle
are plotted in Fig. 2.

241 Germania is a CP-type asteroid (Tholen 1989) with a diameter of 169 km (Tedesco 1989). This asteroid has been observed in two oppositions in 1990 and 1991 (Lagerkvist et al. 1992b; Shevchenko et al. 1992) and now in 1996. Lagerkvist et al. (1992b) observed 241 Germania during five nights in 1990 and the most likely solution that they found for the synodic period for 241 Germania was about 15.2 hours. Shevchenko et al. (1992) observed 241 Germania during four nights in October-November 1991. They found, for 241 Germania, a synodic period of 8.998 hours from their observations. We find a synodic period of 15.51 0.01 hours from our data. This is 18.6 minutes larger than the one proposed by Lagerkvist et al. (1992b). We tried to make composite lightcurves of our observations using these two previously suggested synodic periods, but none of them were consistent with our observations. The composite lightcurves derived for 241 Germania, from our observations, using a synodic period of 15.51 hours are shown in Fig. 3. We also tried to make composite lightcurves of 1990 and 1991 observations of 241 Germania using the synodic period obtained from our observations. Table 3 contains the date of available observations of 241 Germania together with its ecliptic longitude and latitude, the solar phase angle and the mean reduced magnitude observed, , during each night. Figure 4 shows the composite lightcurves derived, using a synodic period of 15.51 hours, from 1990, 1991 and 1996 observations. We have not applied any phase correction to the reduced magnitudes, , however for the nights 13 and 16 of October 1991, where only relative magnitudes are available, we have shifted these relative magnitudes by additional constants of and for the nights 13 and 16 of October, respectively. The different value of the solar phase angle on November 3, 1991 ( ) and on October 12, 1991 ( ) explains the difference in the magnitudes observed those days. The composite lightcurves obtained from 1990 and 1991 observations seem to be well defined with the data points in their rotational phases. This gives us confidence about the synodic period derived for 241 Germania.

Date | Long | Lat | Phase | |

(0 UT) | (1950) | (1950) | (deg) | (mag) |

241 Germania | ||||

22 08 1990 | 301.40 | 5.38 | 9.59 | 8.25 0.02 |

23 08 1990 | 301.27 | 5.39 | 9.93 | 8.64 0.10 |

24 08 1990 | 301.13 | 5.40 | 10.28 | 8.24 0.03 |

26 08 1990 | 300.87 | 5.41 | 10.96 | 8.34 0.07 |

29 08 1990 | 300.52 | 5.42 | 11.95 | 8.39 0.05 |

12 10 1991 | 56.42 | 5.70 | 12.69 | 8.36 0.04 |

13 10 1991 | 56.34 | 5.70 | 12.45 | -2.23 0.02 |

16 10 1991 | 55.96 | 5.71 | 11.43 | -4.60 0.03 |

03 11 1991 | 52.96 | 5.59 | 5.01 | 7.90 0.03 |

04 10 1996 | 18.94 | 8.25 | 4.23 | 8.03 0.04 |

07 10 1996 | 18.32 | 8.24 | 3.43 | 7.95 0.05 |

08 10 1996 | 18.11 | 8.23 | 3.23 | 7.96 0.04 |

The composite lightcurves obtained here for 241 Germania show
two maxima of similar magnitude and two unequal minima per rotational cycle
in all the *uvby* filters.
The largest amplitude in all the *uvby* filters
is of
.
The colour indices do not show any significant variation during the rotational phase of
this asteroid
(see Fig. 3).

The mean values of and found for this asteroid, are in good agreement with the values ( and ) reported by Bowell et al. (1979).

We have used all the available observations of 241 Germania,
from 3 oppositions, to obtain information about its rotational
and shape parameters.
The most probable solution obtained
for the sidereal period, pole and shape is
,
and
or
(
and
)
and values for *a*/*b* of 1.20, for *b*/*c* of 1.5 and
for
of 0.002,
having a prograde sense of rotation (a retrograde solution with
,
might also be possible).
No previous solutions have been determined for this asteroid.
The prograde solution has slightly smaller residuals.
We choose the prograde solution as the most probable.

The observed amplitudes
together with the theoretical amplitudes, at zero-phase angle,
obtained with these solution values of *a*/*b* and *b*/*c* versus the aspect angle
are plotted in Fig. 5. The agreement is very good, however more lightcurves
of 241 Germania would improve the determination of its rotational and shape
parameters. According to the poles presented here, during the next oppositions
in May/June 2000 (
)
and in
December 2002 (
), the lightcurves of 241 Germania will have
small amplitudes, while during the oppositions in September 2001 (
)
and in the February/March 2004 (
), the aspect of 241 Germania
will be close to equatorial, and then, the lightcurves will have
larger amplitudes. Those observations would help to improve the results
presented here.

Figure 5:
Amplitude obtained considering 241 Germania as a triaxial ellipsoid
with a/b=1.2 and b/c=1.5 Amplitudes observed and corrected by a phase factor
= 0.002 |

This object is of S-type (Tholen 1989) with a diameter of 59 km (Tedesco 1989). There are no available lightcurves for this asteroid in previous oppositions.

We find a synodic period for 509 Iolanda of 12.72
0.02 hours.
The composite lightcurves obtained for 509 Iolanda show
two maxima and two minima per rotational cycle
(see Fig. 6).
There are gaps in the coverage of the lightcurve of this asteroid, and only
one maximum and minimum are completely defined.
The largest
amplitude observed in all the *uvby* filters is greater than
.

Figure 6:
Lightcurves and colour indices of 509 Iolanda in
rotational phase. The 0 phase time corresponds to JD 2450361.3211 corrected
for light-time |

The colour indices obtained for this asteroid show large dispersions and no significant variations are detected in the colour indices along the rotational phase. We find a mean value of colour index that agrees with the value, , reported by Bowell et al. (1979). Although the mean value of obtained is smaller than the value, , reported by Bowell et al. (1979), the difference between the value obtained and the one reported is within the large error bars of the determination and thus may not be significant.

There are not enough lightcurves to deduce the sidereal period, pole and shape of 509 Iolanda. For this asteroid, lightcurves taken at different ecliptic longitudes are needed to determine its rotational and shape parameters.

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