Avantor rises after activist investor Engine Capital builds stake

** Shares of life science firm Avantor AVTR.N rise 8% to $12.44

** Activist investor Engine Capital, which owns about 3% of co's shares, sent a letter to the company's board saying it believes co's shares are "deeply undervalued" and called for its board to be refreshed with new directors

** Engine urges Avantor to consider a sale, estimating a potential transaction to value the company's shares between $17 and $19

** Avantor says its board is "actively engaged" in setting and executing strategy that is in the best interest of the company and its shareholders

** "We look forward to an ongoing dialogue with all our shareholders, including Engine Capital" - Avantor

** Up to last close, stock down 45.4% YTD