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ever since the work of edwin hubble , the cepheid period - luminosity ( @xmath0 ) relation is a fundamental tool in determining galactic and extragalactic distances . in spite of its paramount importance , to this day
we still lack firm theoretical and empirical assessment on whether or not chemical composition has any significant influence on the pulsational properties of cepheids .
the debate on metallicity effects on the period - luminosity relation is an old one ( see , for example , the reviews by madore & freedman 1991 and , more recently , feast 2003 ) , but the matter is far from being settled . as an illustration ,
different prescriptions on how to account for the metallicity effect lead to differences as large as 10% in the the distance to the large magellanic cloud ( e.g. groenewegen & oudmaijer 2000 ) , which directly translates into a 10% uncertainty on @xmath1 . to make things worse ,
this effect is _ systematic _ and only a careful calibration of the dependence of the @xmath0 relation on metallicity can provide firmer ground on which to build the astronomical distance ladder .
regrettably , the current understanding of the subject is poor .
theoretical pulsational models by different groups lead to markedly different results . on the one side computations based on _ linear models _
( e.g. chiosi et al 1992 , sandage et al 1999 , alibert et al 1999 , baraffe & alibert 2001 ) suggest a mild dependence of the @xmath0 relation on chemical composition ( less than 0.1 mag at @xmath2 for a change in metallicity between @xmath3 and @xmath4 ) .
this result is challenged by the outcome of the _ non - linear convective models _ ( e.g. bono et al 1999 , caputo et al 2000 ) which find that both the slope and the zeropoint of the period - luminosity relation depend significantly on the adopted chemical composition . again for @xmath2 and the same variation in metallicity as above , they predict a change as large as 0.4 magnitudes in @xmath5 , 0.3 magnitudes in @xmath6 and @xmath7 magnitudes in @xmath8 .
moreover , the change is such that metal - rich cepheids are _ fainter _ than metal - poor ones , again at variance with the results of alibert et al ( 1999 ) and baraffe & alibert ( 2001 ) .
finally , recent calculations by fiorentino et al ( 2002 ) based on non - linear models indicate that the @xmath0 relation also depends on the helium content of the stars .    from an observational standpoint ,
the subject was approached in essentially two ways : a direct measurement of the iron content in nearby , _
i.e. _ bright , cepheids , or by measuring a secondary metallicity indicator in external galaxies known to contain cepheids , under the assumption that they would have the same chemical composition .
regrettably , _ direct metallicity determinations
_ for individual cepheids exist only for a handful of stars .
the most accurate studies are by fry & carney ( 1997 , [ fe / h ] and [ @xmath9/h ] for 23 galactic cepheids ) and andrievsky et al ( 2002 , complete abundance analysis of 77 galactic cepheids ; see also luck et al 2003 ) .
the former find a spread in [ fe / h ] of 0.4 dex , which they argue is real , the latter focus on investigating galactic abundance gradients ( @xmath10  dex / kpc ) .
luck et al ( 1998 ) have measured the iron abundance of 16 cepheids in the large and small magellanic cloud ( lmc and smc , respectively ) with the echelle spectrograph at the ctio 4 meter telescope .
the relatively small size of the telescope forced them to choose only stars at the bright end of the period - luminosity relation , _
i.e. _ long periods , whereas the entire range needs to be sample in order to reach meaningful conclusions on the @xmath0 relation ( e.g. kennicut et al 1998 , caputo et al 2000 ) .
as for the _ indirect measurements _ in external galaxies ( e.g. kochanek et al 1997 , kennicutt et al 1998 ) , which probe a broader range in metallicity than the one available in the galaxy , they find that metal - rich cepheids are _
brighter _ in @xmath5 and @xmath6 than metal - poor ones by @xmath11 to @xmath12 magnitudes per dex in metallicity .
these results have been widely used in the literature , including in the final paper of the hst key project on @xmath1 ( freedman et al 2001 ) .
with the scenario described above in mind , we have collected high quality spectra of an unprecedented dataset of 40 galactic , 22 lmc and 14 smc cepheids with the aim of measuring metallicity directly on a statistically significant sample of stars covering about one order of magnitude in chemical composition .
the galactic stars were observed with feros at the eso 1.5 m telescope on la silla at a spectral resolution of 48,000 with a signal - to - noise ratio of about 70 to 150 . as for the spectra of the magellanic cloud objects
, we used the uves spectrograph on the kueyen telescope on cerro paranal , yielding a resolution of 40,000 and signal - to - noise ratio of about 50 to 70 .
deriving metallicities for cepheids is by no means a trivial task , but we have now carefully analyzed and derived [ fe / h ] for an initial subsample of 14 cepheids in the lmc , 12 in the smc and 6 in the galaxy .
the data reduction procedures and analysis are described in detail in mottini et al ( 2003 ) .
the main result from the subset analyzed so far is summarized in figure  1 . there , we plot the v - band residuals from the standard madore & freedman ( 1991 ) @xmath0 relation as a function of the iron abundance derived from feros and uves spectra .
a positive magnitude difference here means that the star is fainter than the mean @xmath0 relation . for consistency with the adopted @xmath0 relation ,
the lmc is assumed to have a distance modulus of 18.50 .
the smc is considered 0.44  mag more distant ( e.g. cioni et al 2000 ) and the distances to galactic stars were taken from fouqu et al ( 2003 ) , barnes et al ( 2003 ) and tammann et al ( 2003 ) .
photometry and reddening are taken from laney & stobie ( 1994 ) .
the data are binned in metallicity to reflect the typical observational error on our determination ( @xmath13  dex ) .
the magellanic cloud cepheids populate the bins at lower metallicity , the higher [ fe / h ] one containing only galactic stars .
the median magnitude residual in each bin is plotted as a square and the rms and error on the median are shown as boxes and errorbars , respectively .
the magnitude difference plotted on the ordinates can be directly compared to the _ `` c '' _ quantity computed by fiorentino et al ( 2002 ) .
this is the correction to be applied to the distance modulus derived using a universal , metallicity independent @xmath0 relation to take into account the influence of metallicity as derived from their non - linear pulsation models .
the agreement between our data and the predictions by fiorentino et al ( 2002 ) , indicating a strong dependence of the @xmath0 relation on metallicity , is tantalizing , even though the errorbars are still somewhat large . as it can be seen , the error on the inferred distance can be as large as 10% if the effect of metallicity is neglected .
we present preliminary results of an observational campaign devoted at establishing the influence of chemical composition on the cepheid period - luminosity relation . 
 the data are in good agreement with theoretical predictions based on non - linear convective models , suggesting a fairly strong dependence of the period - luminosity relation on metallicity in the sense of more metal rich stars being intrinsically fainter than otherwise expected . 
 our data indicate that the error on the inferred distance can be as large as 10% if the role of metallicity is neglected .    
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