Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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Based on observations collected at the German-Spanish Astronomical Center (DSAZ), Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy.

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...

Based on data obtained at ESO (ESO proposal No. 58.D-0478, 65.H-0253(A)).

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...

Some observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution.

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... $.\!\!{\hbox{$^{\prime\prime}$ }}$ 0

Note that PG 0105+276, which is resolved in three components (see Fig. 1), is an sdO star and does not belong to our sdB sample.

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