Based on observations collected at the Multiple Mirror Telescope on Mt. Hopkins.

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Tables 2, 3 and full Fig. 1 are only available in electronic form at http://www.edpsciences.org

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...I

The 4000 Å break contrast C is defined as (S⁺-S^-)/S⁺, where S⁺ and S^- are the fluxes measured redward and blueward of the break, respectively. Several tests with our population synthesis method have shown that for lower values of C the stellar populations cannot be determined.

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