Light and colour variations of Mira variables in the Small Magellanic
  Cloud
  The goal of this paper is to characterise the light variation properties of
Mira variables in the Small Magellanic Cloud. We have investigated a combined
optical and near infrared multi-epoch dataset of Mira variables based on our
monitoring data obtained over 15 years. Bolometric correction relations are
formulated for various near-infrared colours. We find that the same bolometric
correction equation holds for both the bolometricly brightest and faintest
pulsation phases. Period-bolometric magnitude relations and period-colour
relations were derived using time-averaged values. Phase lags between
bolometric phase and optical and near-infrared phases were detected from the
O-rich (the surface C/O number ratio is below unity) Mira variables, while no
significant systematic lags were observed in most of the C-rich (the C/O ratio
is over unity) ones. Some Miras show colour phase inversions, e.g., $H-K_{\rm
s}$ at its bluest while $J-H$ and $J-K_{\rm s}$ are at their reddest values at
about the bolometricly brightest phase. Their occurrence conditions were
studied but no clear direct or indirect trigger was found. A large NIR colour
change unassociated with stellar pulsation was observed in Miras with long
secondary periods, and its possible explanation is described.
