A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc−1 and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established. Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal‐rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal‐poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal‐rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre‐enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.

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Abstract.A new method, AMORE -based on a genetic algorithm optimizer, is presented for the automated study of colourmagnitude diagrams. The method combines several stellar population synthesis tools developed in the last decade by or in collaboration with the Padova group. Our method is able to recover, within the uncertainties, the parameters -distance, extinction, age, metallicity, index of a power-law initial mass function and the index of an exponential star formation rate -from a reference synthetic stellar population. No a priori information is inserted to recover the parameters, which is done simultaneously and not one at a time. Examples are given to demonstrate and to better understand biases in the results, if one of the input parameters is deliberately set fixed to a non-optimum value.

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Abstract. A quantitative method is presented to compare observed and synthetic colour-magnitude diagrams (CMDs). The method is based on a χ 2 merit function for a point (c i , m i ) in the observed CMD, which has a corresponding point in the simulated CMD within nσ(c i , m i ) of the error ellipse. The χ 2 merit function is then combined with the Poisson merit function of the points for which no corresponding point was found within the nσ(c i , m i ) error ellipse boundary.Monte-Carlo simulations are presented to demonstrate the diagnostics obtained from the combined (χ 2 , Poisson) merit function through variation of different parameters in the stellar population synthesis tool. The simulations indicate that the merit function can potentially be used to reveal information about the initial mass function. Information about the star formation history of single stellar aggregates, such as open or globular clusters and possibly dwarf galaxies with a dominating stellar population, might not be reliable if one is dealing with a relatively small age range.

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We study two different samples of long-period variable Asymptotic Giant Branch (AGB) stars in a field of low and homogeneous extinction towards the Galactic bulge, the Ρ alomar-Groningen field Nr. 3. The samples were selected to study the evolution of the late phases on the AGB. One sample consists of 486 variables (mostly Miras) optically detected and studied by Plaut (1971) and by Wesselink (1987). The other sample is selected from the IRAS Point Source Catalogue and consists of 239 sources. We made additional infrared measurements between 1.2 and 13 μτη for a large fraction of both samples. This information was used to identify the IRAS sources and derive the apparent bolometric magnitudes. The samples of Miras and variable IRAS sources have a similar apparent bolometric magnitude distributions, but are displaced by an amount significantly less than expected from the Mira period-luminosity relation (Feast et ai 1989; 0.3 magnitudes as opposed to 0.6 magnitudes). The surface density distribution along the minor axis of the bulge is the same for both samples. We conclude that both samples have evolved from the same parent population and that they represent different evolutionary stages on the AGB. The IRAS sources with the longer periods (on average 450 days (Whitelock et al. 1991) versus on average 250 days for the optical sample) are the further evolved objects. As the IRAS sources have higher mass loss rates we conclude that mass loss increases during the late stages of the evolution. However, we find indications that in some stars the mass loss process has been interrupted for some time; mass loss could be an intermittent process although its overall rate increases in time. The Miras and the IRAS sources in the bulge have a very similar spatial distribution which also agrees with that found by Blanco (1988) for late type M giants (Fig. 1). The distribution differs considerably from that of the metal-poor RR Lyrae stars. From model calculations in the literature we estimate that the population of long-period variable AGB stars is more than 10 Gyrs old and originates from stars with a Main Sequence mass of about 1 MQ. Unlike Harmon and Gilmore (1988) who estimate the ages * Based on observations collected at the European Southern Observatory (La Silla, Chile) as part of the ESO Key Programme "Stellar Evolution in the Bulge". 291 H. Dejonghe and H. J. Habing (eds.), Galactic Bulges, 291-282.

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