Chemical constituents of the essential oils of Sideritis scardica Griseb. and Sideritis raeseri boiss and heldr. from Bulgaria and Macedonia - E. Kostadinova, D. Nikolova, K. Alipieva, M. Stefova, G. Stefkov, L. Evstatieva, V. Matevski, V. Bankova (My University, 2007)