Context :

Aims : We devise a simple experiment to test the theory that lenticular ( S0 ) galaxies form from spirals whose star formation has been shut down .
An individual galaxy ’ s fading is measured using the globular cluster specific frequency ( S _ { N } ) , defined as the number of globular clusters normalised by the galaxy luminosity .
This is compared with a spectroscopically-derived age estimate .

Methods : We make NTT/EMMI long-slit spectroscopic observations of 11 S0 galaxies at z < 0.006 .
We measure the absorption-line indices , H \delta , H \gamma , Mg b , Fe5270 and Fe5335 within the central r _ { e } / 8 .
By inverting single-stellar population models , luminosity-weighted mean ages , metallicities and \alpha -element abundance ratios are derived .
We estimate the amount of fading a galaxy has undergone by comparing each galaxy ’ s S _ { N } with its deviation from the mean spiral S _ { N } .

Results : Galaxies with higher S _ { N } have older stellar populations .
Moreover , we find that the zero-point and amount of fading is consistent with a scenario where lenticulars are formed by the quenching of star formation in spiral galaxies .
Our data also rule out any formation method for S0s which creates a large number of new globular clusters .
We confirm that previous results showing a relationship between S _ { N } and color are driven by the S _ { N } - Age relation .
Five galaxies show detectable H \beta , [ O iii ] , H \alpha or [ N ii ] emission lines .
However , only in the two youngest galaxies is this emission unambiguously from star formation .

Conclusions : Our results are consistent with the theory that S0 galaxies are formed when gas in normal spirals is removed , possibly as a result of a change in environment .
The on-going star formation in the youngest galaxies hints that the timescale of quenching is \lesssim 1 Gyr .
We speculate , therefore , that the truncation of star formation is a rather gentle process unlikely to involve a rapid burst of star formation .