reduced using the standard IRAM package CLASS. A summary of line parameters at the dust peak is given in Table  \[linepar\] and the corresponding spectra are shown in Fig.  \[spectra\]. The spectra shown in Fig.  \[spectra\] (as well as the values in Table  \[linepar\]) have been derived from data gaussian–smoothed to a resolution of 26in the cases of  and  , but unsmoothed in the case of . These are also the effective resolutions of the  maps shown in Fig.  \[fig1b\] (the two  maps are smoothed to 16, the angular resolution at the (2–1) frequency).

One clear result from Fig.  \[spectra\] is that in L1521F there is evidence for an “infall signature” although less marked than in L1544, where it is attributed to extended infall onto the core (Williams et al. [@Wil99]) or to central infall in a depleted core nucleus (Caselli et al. [@cas02a]). In fact, we see evidence for asymmetric profiles, with the blue peak brighter than the red peak, in  (1–0) (where we derive an optical depth of order 4 in the main component based on our fit to the hyperfine satellites), but the two peaks are not clearly separated as in the case of L1544.

It is 