** Shares of Howard Hughes HHH.N up 9.6% at $78.65 premarket
** Billionaire investor Bill Ackman says on X.com he has written a "letter proposal" to the real estate developer
** Ackman on Sunday asked HHH investors to "watch out for our amended HHH Schedule 13D that we will file with the @SECGov at 6am Eastern, tomorrow morning" in the social media post
** HHH stock up 15% since Ackman's Pershing Square Capital Management, already the biggest owner of Howard Hughes, said it was considering taking co private in August
** Howard Hughes is one of Pershing Square's longest-held investments and Ackman has been personally involved with the company for a decade and stepped off its board in April 2024
** Stock hit a four-month low on Friday, down about 18% from its 2-1/2 year high in November
(Reporting by Medha Singh)
((medha.singh@thomsonreuters.com))