Section 41. Stockholders entitled to vote shall have one vote for each share of stock owned by them and a proportionate vote for a fractional share; provided that in corporations having two or more classes or series of stock, the voting powers of the different classes or series may be fixed in the manner provided by section thirteen or section twenty-six. Capital stock shall not be voted if any instalment of the subscription therefor has been duly demanded under section twenty-two and is overdue and unpaid. Stockholders may vote in person or by proxy. No proxy dated more than six months before the meeting named therein shall be valid and no proxy shall be valid after the final adjournment of such meeting. Notwithstanding the provisions of the fourth sentence, a proxy coupled with an interest sufficient in law to support an irrevocable power, including, without limitation, an interest in the shares or in the corporation generally, may be made irrevocable if it so provides, need not specify the meeting to which it relates, and shall be valid and enforceable until the interest terminates, or for such shorter period as may be specified in the proxy. A proxy with respect to stock held in the name of two or more persons shall be valid if executed by any one of them unless at or prior to exercise of the proxy the corporation receives a specific written notice to the contrary from any one of them. A proxy purporting to be executed by or on behalf of a stockholder shall be deemed valid unless challenged at or prior to its exercise and the burden of proving invalidity shall rest on the challenger.