| Abstract
| - We present $uvby\beta$ CCD photometry of the central region of the double cluster h& χ Persei. We identify ≈350 stars, of which 214 were not included in Oosterhof's catalogue. Our magnitude limit $V=16.5$ allows us to reach early F spectral type and obtain very accurate fits to the ZAMS. We derive reddening values of $E(b-y) = 0.44\pm0.02$ for h Persei and $E(b-y) = 0.39\pm0.05$ for χ Persei. From the ZAMS fitting, we derive distance moduli $V_{0}-M_{V} = 11.66\pm0.20$ and $V_{0}-M_{V} = 11.56\pm0.20$ for h and χ Persei respectively. These values are perfectly compatible with both clusters being placed at the same distance and having identical reddenings. The shift in the main-sequence turnoff and isochrone fitting, however, show that there is a significant age difference between both clusters, with the bulk of stars in h Persei being older than χ Persei. There is, however, a significant population of stars in h Persei which are younger than χ Persei. All this argues for at least three different epochs of star formation, corresponding approximately to $\log t = 7.0, 7.15$ and 7.3.
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