Starlady Sandra’s Adventure : LEADER OF THE BAND,<i> by Fay Weldon</i> ,<i> (Viking: $17.95; 196 pp.; 0-67082440) </i> : POLARIS & OTHER STORIES,<i> b y Fay Weldon (Penguin Books: $6.95, paper; 237 pp.; 0-14-009747)</i>

A double shot of wise woman Fay Weldon’s Special Purgative Tonic for the contemporary heart is what we get with the simultaneous publication of her latest novel and second book of short stories. The flavor of the potion has mellowed over the years since “The Life and Loves of a She-Devil,” but the kick’s still there. You might find yourself looking up from Weldon’s pages at an approaching mate or child and impulsively dropping the usual accommodating response for a frank confrontation. You might even take the novel’s heroine, Starlady Sandra, for a role model and walk out on the whole show, especially if someone as exciting as Jack the Mad Trumpeter is available for company.

But perhaps not. For Sandra is a confection of new and old fantasies: a distinguished astronomer who has discovered a new planet and named it Athena, who’s blonde, in great shape at 42 and the star of her own TV science program. She has forgone childbearing with the best excuse yet--she happens to be the daughter of an insane Gypsy mother and a German SS camp doctor. Hence, she says, “I make myself deaf to the pleas of the unborn. As many as my father brought into existence, I will keep out of it. I will make things even, as the whole universe craves to do. . . . Let him stand outside the wall and breathe his ghostly breath, and wait; I won’t have his children, no I won’t.”

In a weak moment, she has fallen into a boring marriage with Matthew, an ambitious lawyer. But with such a destabilizing background, who can blame her for breaking out and running off to France for a summer of happy sex and music? As for Jack’s own family (weepy wife Anne, teen-age problem daughter and small son), they ought to be used to his escapades, since these occur every summer. That’s the way artistes are, thinks Sandra cheerfully, and she wastes no pity on their women who drudge and cling:

“Women who play mother--who nurture, cosset, bite back harsh words--get left, as if they were the real thing. . . . And the man is right to go. For the woman who lives her life through a man is truly manipulative and dangerous: She has him retired and in slippers, or pottering round the garden in no time at all.”

Sandra won’t live that way (and economically she doesn’t have to). She understands that the sexual arena is ruled by random affinities, competitiveness and opportunity, and takes her chances knowing there’s no security or fairness in it. At the end, pregnant by Jack, she’s thinking about leaving him before he leaves her, and--in a remarkable about-face--becoming a single mother. Here the author plays the Mother Nature role, allowing her heroine to enjoy the free-lancers’ party only for so long, then bringing her back to the procreative fold.

“Polaris & Other Stories” continues Weldon’s exploration of the ways men and women invent to satisfy their complex desires. The title story is the prize piece of the collection. In it, the officers in charge of a nuclear attack submarine are shown managing to hold a difficult balance under the pressure of their dehumanizing jobs and interrupted marriages. The key to that balance turns out to be the practice of concocting exquisite international cuisine at the bottom of the ocean. It’s a charming and persuasive vision, as in the scene where a launch countdown is about to suffer an emergency interruption:

“ ‘Some of the exotic veg, sir, aren’t on board,’ said Percival. ‘We have no aubergines, no fresh chiles and no fresh ginger.’ The captain turned a concerned face towards his crewmen. ‘Of course, sir,’ said Rating Daly, ‘we have powdered chile and powdered ginger: that’s stock issue. But I know how keen you are on the fresh, and it doesn’t solve the aubergine question.’ ”

One of the chief delights in these stories is the accuracy in taking on the manners of a particular institution or of a relationship. And, like the experienced dramatist that she is, Weldon makes entertaining use of conversational repartee. In “Redundant! or the Wife’s Revenge,” a philanderer convalescing from a face lift complains to his wife (to avoid discussion of their marriage): “It hurts to talk.” “It always did, darling,” she observes, and pushes right on with her agenda.

In an age that favors specialization among writers, Weldon has found and held her own line as a smart and wickedly outspoken interpreter of women’s relationships. As any good specialist, she has her devoted readers, for whom she is the one saying what needs to be said; then there are those who find her energizing in small quantities and those who can’t take her at all. Under these circumstances, it feels awkward (or irrelevant?) to take the judicious line and say that it’s a great pity her range both of vision and voice remains so limited. But it is a pity.

Thus, when she offers us Sandra, in “Leader of the Band,” as a professional woman scientist and then fills her head with old-style ruminations on feelings and relationships (with only a glance at electrons and popular biology), she does a disservice to the reality of women who have taken on the discipline of scientific study and the preoccupying life it entails. Moreover, Weldon’s authorial voice, that insistent, buttonholing manner that has long been her trademark, is wearing badly. Her emphatic tags-- well of course, as we know-- end by bullying the reader’s responses to exhaustion. It’s a voice that can seem, sadly, to have been right about the same few things once too often.