Systematic variability in stellarmagnitudes, as derived from profile fitting to CCD images,may
in some instances be due to variable seeing. It is suggested that this happens in cases where the
stars are unresolved pairs, typically with sub-arcsecond separation between the components. It
is shown that the fitting of suitable Generalised Additive Models to time series photometry can
disentangle intrinsic stellar variability and seeing-induced brightness changes. It is possible
that there will be a fixed seeing response associated with a given star which exhibits the effect:
estimation of this response from several long photometric runs is demonstrated.Web of Scienc
