I'm a serial startup founder, and I've been in the position that your CTO and CEO are in right now. It's hard and expensive to get a company off the ground and if you can find someone who will work for you, who's good, and they'll do it for less than they're worth, then awesome. My feeling is that if I'm paying someone 25% less than they're worth, then that makes it easier for me to give them a nice raise once we're "on our feet" financially. But that's a moving target, and it's always tempting to just let things ride. And if someone doesn't ask for equity, maybe I'll just let that ride too, because who wants to give up equity if they don't have to.

I think these guys are making a mistake, though, because they're not just letting you get by making a little less that what you could make on the open market, and they're not just giving you a small amount of equity when you could perhaps demand more. They're playing with fire, because there's a good chance you could go out and find a good job, and by the time they find out you're dissatisfied, it's too late.

But here's what I'm certain of: If, as other commenters have recommended, you have a meetup with them, and you let them know that you feel you're not being adequately compensated, and you've been thinking of starting a job hunt, but that your sense of loyalty dictates that you let them know now, before to get fed up and before you find another job, they'll give you a sweeter deal.

They may not be able to afford to pay you more cash, but they can definitely afford to give you a decent equity stake. I can't say how much would be okay, but 0% is definitely not okay. In this case, you need to turn the tables. Once you've agreed to work for equity in lieu of a full-market salary, you become an investor, and they need to pitch you on the company's future. As for deciding what's appropriate, I'd recommend not thinking in terms of a percent, but have them make the case for the company being worth X amount. If they've raised money from other investors, then that may already be determined. Then you can agree on an amount of equity that's equal to a certain amount of the company's value. If you think you've missed out on $50,000 of compensation so far, then you should ask for that much equity, plus whatever extra you think you should get for the work you'll do in the near future.

Then as the company's fortunes rise, then so will yours. Of course, if you're going to take equity at all, you need to have them make the case, and you need to believe it. Otherwise, keep working at this job long enough to get a new one, and let the chips fall where they may.