Sept 27 (Reuters) - Foreign investors were net buyers of Japanese shares for a fourth straight week, buoyed by big gains in domestic stocks this month.

Hopes for a new political leadership and expectations of an economic rebound from the COVID-19 lows, lead net foreign purchases worth 247 billion yen in the week to Sept. 17, data from Japanese exchanges showed.

Investors purchased 297.02 billion yen (about $2.7 billion) worth of derivatives but sold 50.03 billion yen in cash equity markets.

In the week to Sep. 17, Japanese shares closed higher for a fourth straight week, thanks to the rising vaccination rate, easing infections and expectations of a stronger political leadership bolstering recovery prospects.

The Nikkei share average and the Topix index , both gained about 0.4%.

However, both indexes dropped last week as fears of a default by developer China Evergrande Group, hit Japanese firms including toilet maker Toto and Paint maker Nippon Paint Holding, that are seen vulnerable to a slowdown in China's property market. ($1 = 0.0090 yen)

(Reporting by Gaurav Dogra and Patturaja Murugaboopathy; Editing by Rashmi Aich)