Besides these well-marked classes of long-period, short-period, and
Algol variables, there are some stars which seem to vary spasmodically,
remaining sometimes for months or even for years without any change;
and there are others whose variability is suspected but has never been
confirmed. Useful work might be done by amateurs in trying to decide
the status of these doubtful stars.
It must be borne in mind that red stars are notoriously difficult
objects. Two observers comparing one at the same time with the same
star will often come to opposite conclusions, showing how difficult it
is to compare stars which differ much in colour. Whenever possible, red
comparison stars should be selected to avoid this uncertainty.
The second star on the list below has a very peculiar type of
variation, unlike any other except a northern star, R Coronae, and a
third recently discovered. The spectra of RY Sagittarii and R Coronae
are also peculiar and resemble one another.
The following are bright southern variables suspected to vary, or known
to vary without recognised laws: