Perhaps you should try some spelling variations:
"variable systems" » "evolvable systems", "multivariable systems", "cable systems"
"variable star" » "variable step"
"variable stars" » "variable steps", "variable start"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("variable star") OR ("variable stars"))) OR ("variable systems")).
• Removing quotes may allow a broader search: (((( variable star ) or ( variable stars ))) or ( variable systems )).
• Adding a wildcard symbol may retrieve word variants: (((("variable star") or ("variable stars"))) or ("variable systems"))*.