1 Introduction

The moderately old open cluster NGC 7789 is among those having very rich blue-straggler populations. Ahumada & Lapasset (1995) give a cluster age of 1.6 Gyr, and the number of suspected blue-straggler candidates is reported to be 25. Despite this large blue-straggler population, no quantitative abundance analysis has been performed so far.

Twenty years ago NGC 7789 with its blue stragglers became a paradigm to explain the existence of such stars by assuming ad hoc additional internal mixing which extends the main sequence phase (Saio & Wheeler 1980). Such a putative mechanism was later discussed by Schönberner & Napiwotzki (1994) who determined accurate effective temperatures and gravities for several blue stragglers in NGC 7789. These authors have shown that the existence of blue stragglers does not result from internal mixing processes, but that they rather evolve like normal stars.

In recent years interest in the NGC 7789 blue stragglers (and in blue stragglers in general) has waned because of the failure to find any spectroscopic signature unique to this phenomenon.

Our third paper from this series is devoted to a detailed spectroscopic investigation of blue-straggler candidates in NGC 7789.