 Loss of coherence in opto-mechanical interferometers. Recently, an interesting proposal of testing the superposition principle by resorting to an experimental set-up involving a (mesoscopic) mirror has been advanced (Marshall et al. 2003). This stimulating proposal has led a group of scientists directly interested in Collapse Theories (Bassi, Ippoliti &amp; Adler 2005; Adler, Bassi &amp; Ippoliti 2005) to check whether the proposed experiment might be a crucial one for testing dynamical reduction models versus quantum mechanics. The problem is extremely subtle because the extension of the oscillations of the mirror is much smaller than the localization accuracy of GRW, so that the localizations processes become almost ineffective. However, quite recently a detailed reconsideration of the physics of such systems has been performed and it has allowed to draw the relevant conclusion that the proposal by Adler (2007) of changing the frequency of the GRW theory of a factor like the one he has considered is untenable. 