N.S. if this behaviour is intended, but when I call the Add method of
SeriesCollection object in Excel VBA, the Windows clipboard gets cleared.

This behavior is consistent in Office 10 & 11, Win XP

Calling the Add method for other objects does not make this happen.

A workaround is to call SeriesCollection.NewSeries, and set the properties
of the new series manually. This does not affect the clipboard.