**Table 1:** Comparison of the Hipparcos (Perryman et al. 1997) and the PPM (Röser et al. 1994) star catalogues.
Catalogue	Hipparcos	PPM
No. of stars	118 218	468 586
Stars/deg²	$\sim$ 3	$\sim$ 7.8
Mean err. position	0.77/0.64^a	0.11/0.27^b
Visual Mag.	9.0	13.5

^a $\sigma_{\alpha}$ $\sigma_{\delta}$ in milliarcsec, at J1991.25; ^b $\sigma$ in arcsec,
dependend on the hemisphere, at J1990.

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